Acute electrolytic stimulation of the median raphe (MR) nucleus (MR Stim) between 13:00 and 16:00 h on proestrus blocked ovulation in 10 of the 21 rats tested, whereas all sham-treated (MR Sham, no current) animals ovulated normally (n = 12). MR Stim at 16:00 h on proestrus in individual cannulated rats ‘turned off’ the luteinizing hormone (LH) surge in all animals tested. Unexpectedly, MR Sham treatment also abruptly inhibited the LH surge, although all animals ovulated. Manipulations associated with acute surgical procedures (puncture of the dura mater, ether anesthesia alone) were also found to inhibit the LH surge, with the magnitude of LH inhibition directly related to the severity of the operative procedures performed (p< 0.025). The results suggest (1) that inhibition of ovulation following MR stimulation can be attributed to a response to stress qualitatively similar but more severe than that experienced by the MR sham group, and (2) that inhibitory inputs to the ovulatory LH release mechanism cannot be studied using acute surgical procedures.