Two behavioral tests, locomotor activity and the acoustic startle response (ASR), were utilized to test for dose-addition of cismethrin, a Type I, or deltamethrin, a Type II pyrethroid, with compounds active to the gamma-aminobutryic acid (GABA) receptor complex (picrotoxin, muscimol, and chlordiazepoxide). All compounds produced dosage-dependent decreases in locomotor activity. Muscimol and picrotoxin decreased ASR amplitude, increased ASR latency and decreased ASR sensitization. Results from the interaction studies indicate dose-addition of picrotoxin and deltamethrin on locomotor activity and the ASR. These data produce further evidence for the differences between the in vivo effects of Type I and Type II pyrethroids, and suggest that the in vivo effects of the Type II pyrethroid deltamethrin may be due in part to interaction with the picrotoxinin binding site of the GABA receptor-ionophore complex. In addition, these results are consistent with reported differential effects of the two classes of pyrethroids on the GABA receptor complex. (Copyright (c) 1987 by the American Society for Pharmacology and Experimental Therapeutics.)