Two behavioral tests, locomotor activity and the acoustic startle response (ASR), were utilized to separate the behavioral actions of Type I and II pyrethroids using permethrin, RU11679, cypermethrin, RU26607, fenvalerate, cyfluthrin, flucythrinate, fluvalinate and p,p'-DDT. Dosage-effect functions for all compounds were determined for both figure-8 maze activity and the ASR in the rat. All compounds were administered po in 1 ml/kg corn oil 1.5 - 3 hr prior to testing. The Type I compounds permethrin RU11679, along with p,p'-DDT, increased amplitude and had no effect on latency to onset of the ASR. In contrast, the Type II pyrethroids cypermethrin, cyfluthrin, and flucythrinate decreased amplitude and increased the latency to onset the ASR. Fenvalerate increased the amplitude, had no effect on latency, but unlike the other compounds tested, increased ASR sensitization. Fluvalinate had no effect on any measure of the ASR. The data provide further evidence of the differences between the in vivo effects of low dosages of Type I and II pyrethroids and extend the findings of our previous work to other representatives of the two classes of pyrethroids.