This study examined the effects of various drug treatments (IP injections) proposed to modify central 5-hydroxytryptamine (5-HT) activity on a conditioned suppression of drinking behavior in water-deprived rats. The subjects were trained to drink their daily water requirement during a 10-min session. Intermittent tone periods of 7 s were then introduced, the last 5 s of which the drinking tube was electrified. The animals gradually suppressed tube contacts during the tone to a low constant level within 2 weeks of training. Diazepam increased punished responding dramatically. The 5-HT antagonists methysergide (1 - 18 mg/kg), cyproheptadine (1 - 18 mg/kg), metergoline (0.25 - 2.0 mg/kg) and cinanserin (10 - mg/kg) failed to induce large, reliable increases in punished responding. When a low dose of diazepam was combined with 5-HT antagonists, only one treatment, methysergide at 3 mg/kg, potentiated the anticonflict activity of diazepam. Acute or chronic treatment with PCPA increased behavior suppressed by punishment, but this effect was weak, brief, and poorly related to the depletion of brain 5-HT. LSD (0.3 - 100 microgram/kg) administered 1, 10, or 30 min before the test was ineffective in overcoming suppression by punishment. Mescaline (6 - 30 mg/kg) had no significant effect on punished responding. 5-HTP (18 mg/kg) decreased the number of shocks accepted, but not after pretreating with carbidopa. Pretreatment with carbidopa plus 5-HTP potentiated the anticonflict effect of diazepam. The 5-HT agonist mCPP (0.25 - 2.0 mg/kg) enhanced suppression due to punishment, but only in doses that interfered with unpunished responding. The 5-HT-releasing agent fenfluramine (0.25 - 1.0 mg/kg) did not affect this behavior. Amitriptyline pretreatment in a dose not affecting unpunished behavior (5.6 mg/kg) potentiated the diazepam-induced increase in punished responding. These results are difficult to reconcile with the proposal that suppression of behavior consequent to punishment is related to brain 5-HT activity.