A single dose of clomipramine, 10 mg i.v., or desipramine, 25 mg i.m., was administered to seven healthy young sibling pairs in a randomized cross-over experiment. The response of serum growth hormone, prolactin and cortisol was measured. The main findings were (1) sex differences in the growth hormone and cortisol response to desipramine and (2) a significant genetic component of the prolactin and cortisol response to desipramine as indicated by significantly (p less than 0.05) lower within-pair than between-pair variance in the sibling pairs but not random pairs of the experimental subjects.