Surges of both LH and FSH are secreted on the day of proestrus in the rat; both surges are blocked by pentobarbital (PB), WHICH ALSO BLOCKS OVULATION. The purposes of the present study were (a) to see if antisera to ovine LH or FSH would prevent ovulation from endogenously secreted rat hormones; (b) to see if exogenous ovine LH or FSH alone can cause ovulation in the presence of antiserum to ovine LH or FSH, when endogenous hormone secretion is suppressed by PB. Antisera were assessed for anti-OAAD or anti-hCG augmentation activity against ovine and/or rat pituitary hormones. (a) With respect to tests against endogenously secreted hormones, antisera to LH block ovulation, but antisera to FSH do not, regardless of their potency against exogenous hormones. (b) Antisera to either ovine LH or FSH can prevent ovulation to either exogenous ovine hormone, in relation to their ability to block LH on OAAD bioassay, but not in relation to their ability to block FSH on the hCG augmentation test. However, one antiserum to FSH, which had very low anti-OAAD activity,was able to prevent ovine FSH, but not ovine LH, from causing ovulation, suggesting that not all of the ability of exogenous FSH to cause ovulation is related to intrinsic or contaminating OAAD activity. These data suggest that LH (as defined by OAAD) is the obligatory ovulating hormone in the rat. Additional data on uteirine intralumenal water in the presence of PB blockade indicate that LH, but not FSH, can induce some estrogen secretion, but that either hormone may cause some progesterone secretion.