The inhibitory action of synthetic pyrethroids and some chlorinated hydrocarbon insecticides on the neural calcium-calmodulin-dependent protein phosphatase, calcineurin, was studied using one radiotracer and two colorimetric methods. It was found that all insecticidal Type II pyrethroids (cypermethrin, deltamethrin and fenvalerate) are potent inhibitors of isolated calcineurin from bovine brain. Their IC50 values were approximately 10(-9) to 10(-11) M. By contrast, neither noninsecticidal chiral isomers of these pyrethroids, neuroactive Type I pyrethroids nor neuroactive chlorinated hydrocarbon insecticides showed comparable potencies against this enzyme. To confirm the action of Type II pyrethroid in situ, isolated intact rat brain synaptosomes were incubated with [32P]phosphoric acid and subsequently depolarized in the presence and absence of 0.1 microM deltamethrin. As expected, there was a sharp rise in protein phosphorylation due to the action of calcineurin. Deltamethrin caused a distinct delay in the dephosphorylation process. The results clearly indicate that calcineurin is specifically inhibited by Type II pyrethroids.