Behavioral effects and blood or plasma levels of d-amphetamine, ethanol, cocaine, and diazepam were examined in rhesus monkeys treated chronically with alpha-l-acetylmethadol (LAAM), methadone, or vehicle. Chronic treatment with the opiates failed to alter blood or plasma levels and behavioral effects of d-amphetamine or ethanol. LAAM-maintained monkeys were somewhat less sensitive to rate-decreasing effects of cocaine on schedule-controlled responding, but cocaine plasma levels and half-lives generally did not differ across the chronic treatment conditions. Behavioral depression after diazepam was prolonged substantially in LAAM- and methadone-maintained monkeys, but blood levels of diazepam and metabolites were not increased prolonged in those animals. Naloxone partially antagonized the residual depression LAAM- and methadone-maintained monkeys 24 hr after diazepam, but had no effect on the weaker sesidual depression in vehicle-maintained aniamals. Thus, diazepam appeared to interfere with the metabolic inactivation of the opiates. One LAAM-maintained monkey showed recurrent episodes of LAAM overdose and eventually died during the course of the study.