Time-dependent ovulation-blocking effect of ether anesthesia was investigated using female Wistar-Imamichi rats showing a 4-day cycling rhythm. A 6-hr (1400-2000) and 8-hr (1400-2200) anesthesia with ether vapor on proestrus caused ovulation blockade in 85% and 100% of the rats, respectively. On the other hand, pentobarbital injections at 1400 and 1500 completely blocked the spontaneous ovulation, while the injections at 1600 and 1700 did not. Ether (1400-2000) and pentobarbital anesthesia decreased the mean serum LH levels at 2000 on proestrus. The LH levels were 13.26 ng/ml for the control, 0.36 ng/ml for the ether group and 0.56 ng/ml for the pentobarbital group, respectively. These findings indicated that ether and pentobarbital similarly suppressed the LH surge, but difference between ovulation-blocking effects by ether and pentobarbital is possibly due to factor(s) other than suppression of the LH surge.