The role of the peripheral parasympathetic nervous system in the development of hypertension was investigated in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. Animals were 5-7 weeks old, anesthetized, and in the open-chest condition. The decrement in heart rate evoked by parasympathetic nerve stimulation (62 +/- 8 beats/min) in SHR was greater (p < 0.01) than that in WKY rats (23 +/- 4 beats/min). Furthermore, the decrease in heart rate (73 +/- 9 beats/min) in response to combined stimulation of sympathetic and parasympathetic nerves in SHR was greater (p < 0.05) than that in response to vagal stimulation alone. The extent of the interaction of sympathetic and parasympathetic nerves was calculated as the difference between the decrease in heart rate during combined stimulation and that during vagal nerve stimulation alone. The extent of the interaction in SHR (-11 +/- 5) was not significantly different from that in WKY rats (-8 +/- 3 beats/min). Therefore, the influence of the peripheral parasympathetic nervous system in the early phase of hypertension may be greater than that in the normotensive state. Interaction between the two branches of the autonomic nervous system may occur as accentuated antagonism originating in the early phase of hypertension. The interaction during the early phase of hypertension may not be different in extent from that of the normotensive state.