Levels of testosterone (T) and LH in the peripheral serum of male and female rabbits were measured and compared following coitus. Blood was collected by heart puncture from restrained, unanesthetized animals of both sexes. In male rabbits, basal serum T levels were highly variable, ranging from 131 to 12,149 pg/ml, and if low preceding coitus they tended to rise; whereas, if high, they usually dropped as they did in nonmated males subjected to repeated heart punctures. In contrast, basal serum LH levels in males were quite constant (mean +/- SE, 1993 +/- 152 pg/ml) and were not significantly altered after coitus unless blood T levels had been drastically lowered by two priming doses of estradiol benzoate. In intact does, on the other hand, copulation which resulted in ovulation induced an approximately 20-fold increase in serum LH concentration which was sustained for about 4 hr. Postcoital elevations in serum LH also occurred in estrogen-primed intact and estrogen-primed ovariectomized does. Under the conditions of our experiments, the parallel elevations in serum LH and T observed postcoitally in the female rabbit could not be demonstrated in the male.