Ovulation was studied in adult rats after treatment with anesthetic doses of urethane on the proestrus of the 4-day cycle. The critical period for LH release was examined by barbiturate sedation and occurred between 14.00–16.00 hr (Photoperiod: 10 hr darkness centred on midnight). Urethane, at 1.2 g/kg, blocked ovulation in about 50% of rats when given between 6.00–14.00 hr on proestrus and allowed only partial ovulation (1–9 oocytes) in many of the remainder. Ovulation was not blocked by treatment after 16.00 hr, though there was a slight reduction in the number of oocytes released. A tranquilizing dose of 0.6 g/kg failed to block ovulation. The threshold dose of exogenous LH for ovulation after urethane at 12.00 hr was 500 ng/rat; the threshold after barbiturate blockade was slightly higher at 1 μg/rat. This, combined with the fact that small doses of LH produced a doseresponse effect in terms of the number of oocytes released, suggests that partial ovulation after urethane may be related to a small release of endogenous LH. Hypophysectomy at 18.00 hr prevented ovulation in rats anesthetized with urethane at 10.30 hr, though it had no effect on the ovulation of controls. Ovulation occurred in the experimental treatments when hypophysectomy was delayed until 20.00 hr. This apparent delay in LH release was correlated with a similar delay in the time of ovulation. The present experiments therefore provide evidence that urethane, given in anesthetic doses before the critical period, blocks the ovulatory release of LH or delays and reduces the release of the hormone. (Endocrinology90: 1594, 1972)