The effects of NAK 1901 (Pentafluorbenzyl (1R, cis)-3-(2,2-dichlorovinyl)-2,2-dimethyl-cyclopropane-carboxylate) and cypermethrin ((S,R)-alpha-cyano-3-phenoxybenzyl-2,2-dimethyl (1R, 1S, cis, trans)-3-(2,2-dichlorovinyl) cyclopropane-carboxylate) (RU 24 501) on amplitude and prepulse inhibition of the acoustic startle reflex were studied in male Wistar rats. NAK 1901 (0, 1, 2.5, 4 mg/kg p.o.) enhanced the amplitude of the startle reflex in a dose-dependent way. Startle latency was not affected. Cypermethrin (0, 0.5, 1, 2 mg/kg p.o.) had no effect on the amplitude or the latency of the startle reflex. Both NAK 1901 and cypermethrin administration produced a dose-dependent increase in toxic signs and a dose-dependent increase in weight loss during the experimental session. None of the pyrethroids affected prepulse inhibition of the startle reflex. Because the neural substrate of the inhibitory processes involved in prepulse inhibition are probably of supraspinal origin, it is suggested that these substrates are not affected by pyrethroids.