Electrolytic lesions were placed in the ventrolateral midbrain and their effects on lactational performance and sexual behavior were assessed. The neuronal injury, situated just above the lateral tip of the substantia nigra in the lateral tegmentum, caused an immediate impairment in lactational performance as measured by the weight gain of the offspring, and interfered also with the hormonal induction of sexual receptivity and proceptivity. In similarly lesioned male rats, mounting behavior was virtually absent, but nociceptive thresholds and locomotor activity in the open-field remained unaltered. It seems likely that some, but not all, of these effects were due to disruption of a pathway carrying somatosensory information relevant for lactation and sexual behavior.