Rats of the high-drinking AA line were given 1 mg/kg naltrexone (NTX) or vehicle orally with a stress-free procedure just before 1 h of access to 10% ethanol daily for 8 days and again, 8 h later on the first 7 days. Forebrain homogenate binding studies using 0.03-6.00 nM [3H] naloxone were conducted from 1 to 4 days following treatment. NTX significantly suppressed alcohol intake, with the effect becoming progressively greater over days and continuing during the post-treatment period. Saturation binding studies in brain homogenate revealed that NTX had increased the B(max) for opioid receptors by 93%, 74%, 49%, and 28%, respectively, from post-treatment days 1 to 4 without altering K(d). B(max) was negatively correlated (r=-0.510, p=0.008) with alcohol intake during the preceding hour, but in control rats, it was positively correlated with changes in alcohol intake over time (r=+0.790, p=0.020). These results are consistent with the hypothesis that opioid receptors mediate reinforcement from alcohol and that NTX reduces subsequent alcohol drinking by extinction. Opioid receptor upregulation can develop simultaneously with suppression of drinking and may partially counteract the clinical benefits from NTX in the treatment of alcoholism.