The effects of autonomic drugs on seminal emission and ejaculation were investigated in mongrel dogs. Changes in the posterior urethral pressure (posterior urethrogram) and the volume of seminal emission during continuous hypogastric nerve stimulation were measured in test animals. The seminal emission induced by hypogastric nerve stimulation caused continuous rise in the posterior urethral pressure. When the pressure reached the maximum level, rhythmic alterations of the pressure which was considered, to be a phenomenon associated with ejaculation occurred. The drugs were administered into the abdominal aorta just above its bifurcation. Tetrodotoxin (1-5 μg) abolished both seminal emission and rhythmic alterations of the posterior urethral pressure. Phenylephrine (10-100 μg) and methoxamine (10-30 μg) increased markedly the seminal emission and induced the rhythmic alterations. Phentolamine (1-10mg) and phenoxybenzamine (1-10mg) abolished both seminal emission and the rhythmic alterations. Administration of isoproterenol (10-30 μg) or propranolol (1-10mg) affected neither seminal emission nor rhythmic alterations. Acetyl-choline (1-10 μg) or atropine (1-10 μg) did not show any definite effect on both seminal emission and the rhythmic alterations. No significant effect of DMPP (10-30 μg) on seminal emission and rhythmic alterations were observed. No significant change in seminal emission was caused by hexamethonium (100-500 μg). The rhythmic alterations were not affected by this agent in many dogs. From these results it is concluded that both seminal emission and ejaculation are predominantly under the influence of adrenergic nervous system, particularly through an a-adrenergic receptor mechanism