Cigarette smoking increased during administration of buprenorphine, an opioid mixed agonist-antagonist, in comparison to drug-free baseline in seven heroin addicts maintained on buprenorphine for 24 days (P less than 0.01-0.001). Ascending buprenorphine doses (0.5 - 8.0 mg/day) were associated with significant increases in cigarette smoking at doses of 2.0 mg/day sc and above. Cigarette smoking during 10 days of buprenorphine maintenance at 8 mg/day was significantly higher than during the buprenorphine induction phase (P less than 0.01). Six subjects given placebo buprenorphine over 14 days showed no change in cigarette smoking. The placebo group self-administered heroin for 10 days, and cigarette smoking increased significantly during heroin use (P less than 0.001). The rate of cigarette smoking defined by intercigarette intervals was highest during the 10 days of high-dose buprenorphine maintenance or placebo plus heroin self-administration. Both groups requested significantly more cigarettes at intervals of 0-10, 11-20, and 21-30 min than during the drug-free baseline. These data confirmed previous findings that opioid agonist administration is associated with increased cigarette smoking and suggest that buprenorphine has primarily agonist effects on cigarette smoking.