SUMMARY: 1. Stimulation of the hypogastric nerves in adult male rabbits under sodium pentobarbitone anaesthesia produced immediate contractions of the cauda epididymidis, vas deferens and seminal vesicle, with emission of semen. Kymographic records were obtained of the contractions of the seminal vesicle. 2. Intravenous injections of 1–5 μg adrenaline gave similar effects, but with latencies of 7–30 sec. Measured by its action on the seminal vesicle, adrenaline was two to five times more potent than noradrenaline. 3. Electrical stimulation of the dorsal, lateral or posterior areas of the hypothalamus evoked an immediate contraction of the seminal vesicle followed by a delayed contractile effect. Contractions of the cauda epididymidis and vas deferens also occurred. 4. The immediate response of the seminal vesicle to stimulation of the hypothalamus was abolished by section of the hypogastric nerves, and the delayed effect by adrenalectomy. After these interferences the two types of response could be simulated by stimulation of the peripheral end of the cut hypogastric nerves, and by injection of 1–5 μg adrenaline, respectively. 5. Neither electrical stimulation of the neurohypophysis, nor injection of oxytocin or vasopressin induced a contraction of the seminal vesicle. 6. The results are discussed with reference to the influence of sexual excitation at coitus on the volume and quality of ejaculated semen.