Spontaneous excitatory junction potentials (e.j.p.s) from the circular smooth muscle cells of the guinea-pig seminal vesicle were recorded intracellularly. Their mean amplitude and frequency were 2.6 mV and 26.6/min, respectively. Occasionally large e.j.p.s were recorded. The circular smooth muscle of the tissue generated the action potential spontaneously, which was accompanied by a twitch-like contraction. The e.j.p.s evoked by field stimulation were also recorded. These potentials in response to the initial several stimuli of a train showed a progressive increase in amplitude. The ratio of the later e.j.p. to the initial e.j.p. ranged from 1.0 to 2.5. The summation of successive e.j.p.s was observed. The rate of depolarization caused by the summation was larger as the frequency of stimulation was higher. The action potential was evoked at a certain level of depolarization due to the summation. In some cells, after reaching the maximum depolarization, the membrane was gradually repolarized during the successive stimuli. In a cell having such properties the action potential could not be evoked. The spontaneous and evoked e.j.p.s were partially blocked in the presence of phentolamine (10(-5) g/ml). Tetrodotoxin (10(-6) g/ml) inhibited the initiation of the evoked e.j.p.s. The evoked e.j.p.s were suppressed by yohimbine (5 X 10(-5) g/ml) and phenoxybenzamine (10(-5) g/ml) but not affected by propranolol (10(-5) g/ml). These results suggest that the spontaneous e.j.p.s in the circular smooth muscle are induced by the action of neurotransmitter released from the nerve endings in the tissue and that the synapse is the alpha-excitatory adrenergic.