The study was designed to determine and compare the acute effects of the enantiomers of mu, kappa and sigma opioid agonists on the cortical EEG with the spectral analysis technique. The relative ability of naloxone to antagonize such effects was also assessed. Adult female Sprague-Dawley rats were implanted with chronic cortical EEG and temporalis muscle EMG recording electrodes, and with permanent indwelling external jugular cannulae. (-)-Methadone(mu agonist) produced increases in spectral power over the zero to 10Hz range, while (-)-ketocyclazocine (kappa agonist) produced increases in the 5-8 Hz band as a predominant peak. The(+) enantiomers of methadone and ketocyclazocine were inactive. The drug (+)-SK-10,047 (sigma agonist), produced a predominant spectral peak in the 7-9 HZ band that was associated with behavior that suggested psychotomimetic effects. The effects of morphine (mu agonist) on EEG and EEG power spectra were more sensitive to antagonism by naloxone than those produced by ketocyclazocine. The effects of (+/-)-SKF-10,047 and (+)-SKF-10,047 were not antagonized by 10 mg/kg of naloxone, while the effects of (-)-SKF-10,047 were partially antagonized by 10 mg/kg of naloxone. These findings further delineate the specificity of the differential effects of mu, kappa and sigma opioid agonists on the EEG and EEg power spectra in the rat.