This study evaluated the effects of i.v. cocaine, hydromorphone and their combination, and assessed the ability of oral naltrexone, an opioid antagonist, to modulate these effects. Volunteers with cocaine and heroin abuse histories (n = 8) participated in this placebo-controlled, cross-over study while residing on a closed research unit. Daily treatment with capsules containing placebo or naltrexone in ascending doses (3.125, 12.5, 50 and 200 mg) were given for 7-day periods. In thrice weekly experimental sessions, cocaine, hydromorphone and their combination were given in random order. Drug doses were given in an ascending order 1 hr apart as follows: cocaine at 0,20 and 40 mg, hydromorphone at 0, 1.5 and 3.0 mg, and the combination of 0 and 0 mg, 20 mg cocaine and 1.5 mg hydromorphone and 40 mg cocaine and 3.0 mg hydromorphone. Hydromorphone and cocaine produced distinct pharmacodynamic profiles, and the combination produced effects similar to both drugs. In some cases, the magnitude of effects produced by the combination was greater than that produced by either drug alone. Naltrexone produced dose-related blockade of hydromorphone effects, but did not after any of the physiological or subjective effects of cocaine. All naltrexone doses partially attenuated the effects of the combination and this appeared to be attributable to selective opioid blockade. These data do not support the use of naltrexone as a treatment for cocaine abuse, but suggest it may be useful for treating patients with concurrent cocaine and heroin abuse.