To determine the involvement of the midbrain in the control of the proestrous surge of gonadotropins and prolactin, various transections were acutely placed in proestrous rats. Half-dome shaped transections which interrupted the neural fibers in the ventromedial part of the midbrain (VMM transection), as well as lesions in the ventral tegmental area, blocked ovulation and the proestrous surges of gonadotropins and prolactin whereas sham transections or the other transections, either dorsal or lateral to VMM transections, had no effect on ovulation. VMM transections, however, affected neither the postcastration rise in serum gonadotropins in males nor the secondary rise in serum FSH in estrous females. These results suggest that in the ventromedial part of the midbrain there exists a common neural mechanism which controls the proestrous surges of gonadotropins and prolactin.