1. The effects of coitus on heart rate, blood pressure and plasma concentration of noradrenaline were studied in six male rabbits. 2. During coitus, heart rate rose from 228 +/- 6 to 324 +/- 8 beats/min (mean +/- SEM; n = 24). Mean blood pressure rose from 87 +/- 3 to 165 +/- 4 mmHg, and plasma concentration of noradrenaline from 2.07 +/- 0.21 to 5.02 +/- 0.26 nmol/l. The maximum heart rate did not occur until after the peak in blood pressure. 3. Propranolol had no effect on the power expended during coitus, but there was a dose dependent fall in the changes in heart rate, blood pressure and plasma concentration of noradrenaline.