Cypermethrin is widely used in agriculture as a type II pyrethroid insecticide. 1 Voltage-dependent sodium channels in nerves appear to be major sites of pyrethroid action. 2 -6 The primary effect of pyrethroids is the closing of the sodium activation gate in nerve cells. In order to characterize the peripheral nerve effects of cypermethrin, we investigated frog sciatic nerve in vitro using the extracellular recording technique. 4,5 Twenty Rana Cameroni frogs ranging in weight from 52 to 65 g were used in the experiments. Sciatic nerves from frogs were removed and placed in frog Ringer solution at room temperature. The Ringer solution had the following composition (mM): NaCl, 111.87; KCl, 2.47; CaCl 2 , 1.08; NaHCO 3 , 2.38. After the nerves were kept in Ringer solution for 1 h, they were placed in plexiglass nerve chambers containing Ag/AgCl electrodes. The width of the chamber was 5 cm and its length was 15 cm. The space between electrodes was 0.5 cm. The nerves were stimulated with Ag/AgCl electrodes. Single square supramaximal pulses of 0.5 ms duration were used to stimulate the nerves. 7 The nerve compound action potentials were recorded before and after cypermethrin treatment using the BIOPAC MP 100 Acquisition System Version 3.5.7 (USA). In the experimental group, the isolated nerves were treated with 10 -5 M cypermethrin (Hektaş) and dissolved in Ringer solution for 15, 30, 45 or 60 min. Compound action potentials were recorded from each nerve before (control group) and after (experimental group) cypermethrin treatments (n = 15). The sciatic nerve experiments were investigated with light microscopy and standard histological procedures for nerve. The data were analysed by ANOVA using the single factor repeated measurement design with a significance level of P < 0.05. Parameters of the experimental group exceeding ±2 SD of the control group were considered abnormal. 8 Preliminary experiments were carried out to determine effective cypermethrin concentration. Cypermethrin concentrations between 10 -9 and 10 -4 M were tested. The 10 -5 M concentration was chosen for further testing because it produced the most significant changes on compound action potential in the isolated frog sciatic nerve.