The effects of acute and chronic ethanol intake on the startle response was examined in male rats. Ethanol given IP produced a dose-dependent decrease in the amplitude of the startle response measured 30 min later. With a dose of 1 g/kg, the effect was evident at 15 min and had recovered substantially by 60 min. The effect of ethanol on the startle response was potentiated by pretreatment of the animals with pimozide, haloperidol, and p-chlorophenylalanine but not by propranolol, phenoxybenzamine, alpha-methyltyrosine, or pargyline. After 3 weeks on an ethanol-containing diet, the startle response was greater than that shown by rats on the control iso-caloric, sucrose-containing diet. After ethanol withdrawal, the startle response was further increased, with a peak about 9 to 12 hr after discontinuation of ethanol; thereafter, the response declined. This time course of heightened startle response during ethanol withdrawal corresponds to the time course of the activation of noradrenergic neurons during withdrawal. It appears that dopaminergic and serotonergic neurons are involved in the mediation of the startle response in rats.