The prohibition of castrating a human or animal:
A. The prohibition:
Male humans/animals: It is [Biblically] forbidden to destroy [i.e. castrate] the reproductive organs of any male creature, whether human, animal, bird [or even fish]. This prohibition applies whether the species is Kosher or impure. This prohibition applies whether in Israel or the Diaspora. One who transgresses this prohibition is liable for the penalty of lashes.
Female humans/animals: It is [Rabbinically] forbidden to destroy the reproductive organs of a female creature, whether human or other creature, although one who does so is exempt from lashes.
It is forbidden to castrate the reproductive organs of any creature, whether human, animal, bird or fish, whether male or female. One who castrates a male creature is Biblically liable for lashes. It is forbidden to further castrate even an already castrated creature.
May one have his pet sterilized/neutered/spayed?
Male pet: It is forbidden to sterilize a male pet through the removal of his organs, or through chemical sterilization. This applies whether one is doing it himself or is having someone do it for him.
Female pet: It is forbidden to sterilize a female animal through removal or destruction of a reproductive organ. This applies whether one is doing it himself or is having someone do it for him.
Coming up in future Halachos under this topic:
- May one do chemical sterilization to a human or animal
- Is a gentile prohibited from performing castration on his own animal
- May one sell an animal to a Gentile who will castrate it
 Michaber E.H. 5/11; Rambam Issurei Biyah 16/10; Shabbos 110b; Vayikra 22/24: “Ubeartzechem Lo Sasu.”
 Birkeiy Yosef 5; Sheilas Yaavetz 111, brought in Pischeiy Teshuvah 5/8
 The reason/source: The verse [Vayikra 22/24] states “Ubeartzechem Lo Sasu” and from here we learn of the prohibition against castration. [Shabbos ibid; Rambam Issurei Biyah 16/10] The reason behind this prohibition is because G-d created the world in perfection and desired the continuity of each of his creations and commanded them to procreate. One who castrates a creature reputes G-d’s creation and is doing an act of destruction to his beautiful world. [Sefer Hachinuch Emor 291]
 Michaber ibid “One who castrates a female is exempt from lashes although it is forbidden to do so”; Rambam Issurei Biyah 16/11; Magid Mishneh on Rambam ibid; Shabbos 111a “Ela Beisha”; Toras Kohanim on Vayikra 22/24 in name of Rebbe Yehuda “There is no castration by females”
Other opinions: Some Poskim rule there is no “castrating” prohibition to castrate a female human/creature, although it is forbidden to cause her pain against her will, or for no need, due to a separate prohibition of Tzaar Baalei Chaim. [Taz 5/6 in interpretation of opinion of Rebbe Yehuda ibid]
 So is the general rule of “Patur Aval Assur” that it contains a Rabbinical prohibition. [See Rambam Shabbos]
Other opinions: Some Poskim rule that the Michaber and other Poskim rule like the first opinion in Toras Kohanim ibid that the prohibition is Biblical even by a female, although one is exempt because they are not included in the Lo Sasei of Lo Sasu. [Gr”a ibid]
 The reason: Castrating a female does not carry Biblical liability being they are not commanded in Peru Urevu. [See Rashi Shabbos ibid] Alternatively, the reason is because their reproductive organs are inside the body and hence cannot be destroyed with direct action. [Taz 5/6] Nevertheless, it is still forbidden to castrate them as even women are commanded to populate the world due to the verse “Lo Letohu Bara Ela..” [Perisha 5; The Taz 5/6 negates this explanation of the Perisha, as if so then it would be forbidden for her to take chemical sterilization.]
 Michaber E.H. 5/14
 Literally “Kutti”
 The reason: This is forbidden due to Amira Lenachri, as every matter in the Torah that is forbidden for a Jew to perform is likewise forbidden for a Jew to ask a Gentile to do for him. [Beis Shmuel 5/16; Beir Hagoleh; Admur 243/1; 343/5; 450/20; Ribis 72; Sheila Usechirus 29; Michaber C.M. 338/6 [regarding muzzling animal]; Question brought in Bava Metzia 90a and above Poskim rule stringently-see Biur Hagr”a 5/32] Alternatively, the reason is because it is Biblically forbidden for even a gentile to destroy the reproductive organs of a human/animal, and it is part of the Nohadite laws. [Beis Shmuel 5/16; Bava Metzia 90a]