1. The effects of the essential oil of Croton nepetaefolius (EOCN) and its major constituent, 1,8-cineole, on the compound action potential (CAP) of nerve were investigated. 2. Experiments were performed in sciatic nerves dissected from Wistar rats, mounted in a moist chamber and stimulated at a frequency of 0.2 Hz, with electric pulses of 100 micros duration at 20-40 V. Evoked CAP were displayed on an oscilloscope and recorded on a computer. The CAP control parameters were as follows: peak-to-peak amplitude 8.1 +/- 0.6 mV (n = 15); conduction velocity 83.3 +/- 4.2 m/s (n = 15); chronaxie 58.0 +/- 6.8 msec (n = 6); and rheobase 2.8 +/- 0.1 V (n = 6). 3. Lower concentrations of EOCN (100 and 300 microg/mL) and 1,8-cineole (153 and 307 microg/mL; i.e. 1 and 2 mmol/L, respectively) had no significant effects on CAP control parameters throughout the entire recording period. However, at the end of 180 min exposure of the nerve to the drug, peak-to-peak amplitude was significantly (P < 0.05) reduced to 27.4 +/- 6.7 and 1.7 +/- 0.8% of control values by 500 and 1000 microg/mL EOCN, respectively (n = 6), and to 76.5 +/- 4.4, 70.0 +/- 3.9 and 14.8 +/- 4.1% of control values by 614, 920 and 1227 microg/mL (i.e. 4, 6 and 8 mmol/L) 1,8-cineole, respectively (n = 6). Regarding conduction velocity, at the end of the 180 min exposure period, this parameter was significantly reduced to 85.8 +/- 7.3 and 48.7 +/- 12.3% (n = 6) of control values by 500 and 1000 microg/mL EOCN, respectively, and to 86.4 +/- 4.5 and 76.1 +/- 5.2% (n = 6) by 920 and 1227 microg/mL 1,8-cineole, respectively. Chronaxie and rheobase were significantly increased by the higher concentrations of both EOCN and 1,8-cineole. 4. It is concluded that EOCN and its main constituent 1,8-cineole block nerve excitability in a concentration-dependent manner, an effect that was totally reversible with 1,8-cineole but not with EOCN. This suggests that other constituents of EOCN, in addition to 1,8-cineole, may contribute to the mediation of this effect of EOCN.