In rats displaying 4-day estrous cycles, pentobarbital (PB) (3 mg/100 g body wt) administered at 1400 on proestrus blocked ovulation in 10/10 animals. The above treatment plus cardiac puncture (CP) at 1600 proestrus (1 mlblood withdrawn) resulted in 11/32 rats ovulating by 1000 the next morning. Serum LH was higher in the sample takea at 1600 proestrus among thePB-CP ovulators than the PB-CP nonovulators. Pituitary LH was decreased in PB-CP ovulatorson the morning of estrus, while PB-CP nonovulatorshad high pituitary LH content, similar tolevels seen in PB-blocked rats not subjected toCP. Serum FSH in PB-CP ovulators was high on the morning of expected estrus, similar to levelsin the ovulating saline controls. These data suggestthat there may have been a release of gonadotropinafter the 1600 proestrous CP sufficient inamount to cause ovulation in a number of animals. Serum LH values at the 1600 proestrousCP suggest that the reversal of PB blockade afterCP was most effective in those rats which alreadyshowed less suppression of LH by PB. The CPprocedure may have stimulated progesterone secretion,since uterine water content on expectedestrus was reduced in 8/21 PB-CP nonovulatorsas well as in all of the PB-CP ovulators. (Endocrinology94: 526, 1974)