Ten pyrethroids affected binding of [³H]perhydrohistrionicotoxin ([ ³ H]H ₁₂ ‐HTX) to the channel sites of the nicotinic acetylcholine (ACh) receptor/channel of the electric organ of the electric ray, Torpedo ocellata, and inhibited ⁴⁵ Ca²⁺ flux through the receptor's ionic channel. Most pyrethroids stimulated binding of [ ³ H]H₁₂‐HTX to the channel sites in 30 s, in absence of carbamylcholine, with little or no effect on binding of [ ³ H]ACh to the receptor sites, which suggests that the pyrethroids are binding to a third kind of site. However, in presence of carbamylcholine, all pyrethroids inhibited binding of [ ³ H]H₁₂‐HTX, with esters of cyclopentenolone more potent, and generally more rapid in doing so, than esters of α‐cyano‐3‐phenoxybenzyl alcohol. Changes in the acidic moiety of the pyrethroid had little effect. Kadethrin, whose alcohol moiety is 5‐benzyl‐3‐furylmethyl, was the most potent pyrethroid in stimulating [ ³ H]H₁₂‐HTX binding (in absence of carbamylcholine) in 30 s (18‐fold) and in inhibiting it in 120 min. The pyrethroids were more potent in their modulation of [ ³ H]H₁₂‐HTX binding at lower temperatures. Inhibition of receptor binding and receptor‐regulated ion transport by concentrations of pyrethroids similar to those at which they affect nerve conduction suggests that the nicotinic ACh receptor may be an additional target for the toxic action of pyrethroids.