The release of LH in ovariectomized, estrogen-primed rats exposed to male or female rats was studied. The concentration of LH in the serum was measured in blood samples obtained by an indwelling jugular cannula. A blood sample was taken at 12.00 h and then every hour up to 18.00 h following exposure to another rat on the opposite side of a double wire mesh screen. LH in the serum of control non-exposed rats showed a small rise between 16.00 and 18.00 h as compared to earlier values. Rats exposed to intact or castrated male rats exhibited an enhanced release. Exposure to ovariectomized rats failed to induce any change in the release of LH but exposure to ovariectomized, estrogen-primed rats produced a significant increase. No effect on LH release was seen in animals exposed to a female diestrous rat or to a pregnant rat. Rats exposed to an empty cage which had been soiled by housing a male rat for 3 days, showed an enhanced release of LH similar to those exposed to a cage containing the male rat, indicating the importance of olfactory stimuli in the response. Ovariectomized rats whose vomeronasal organs had been removed or whose accessory olfactory bulbs were lesioned, failed to show any effect on the release of LH when exposed to a male rat. It is concluded that olfactory stimuli arising from male or female rats are capable of modulating the release of LH in female rats and that perception of these stimuli involves the vomeronasal organ-accessory olfactory bulb system.