1-(1-Naphthyl)piperazine (1-NP) has been reported to have serotonin antagonist properties at the 5-HT2 subtype of receptor, and it has been suggested that it may have agonist actions at the 5-HT1 site. In the present experiments, the effects of 1-NP alone and in combination with a variety of 5-HT agonists, were studied in squirrel monkeys performing under a number of reinforcement schedules. The phenalkylamine hallucinogen 4-bromo-2,5-dimethoxyamphetamine (DOB, 0.01-0.3 mg/kg), which is thought to have predominant actions at 5-HT2 sites, reduced responding under fixed-interval (FI) schedules of presentation of food, and these decreases were blocked by 1-NP (0.3-1.0 mg/kg) or by the selective 5-HT2 antagonist, ketanserin (0.3 mg/kg). 1-(1-Naphthyl)piperazine also antagonized the decreases in responding produced by quipazine (0.1-5.6 mg/kg), another agonist with predominant 5-HT2 actions. 1-(m-Chlorophenyl)piperazine (mCPP, 0.1-3.0 mg/kg) and 1-(m-trifluoromethylphenyl)piperazine (TFMPP, 0.1-3.0 mg/kg), both thought to act primarily at 5-HT1 sites, also decreased responding and this effect was blocked by methysergide and by 1-NP, but not by ketanserin. The effects of 1-NP (0.3-5.6 mg/kg) given alone were not like those of mCPP or TFMPP. 1-(1-Naphthyl)piperazine produced moderate increases in responding under shock-avoidance schedules, whereas only decreases in responding were seen after mCPP and TFMPP.(ABSTRACT TRUNCATED AT 250 WORDS)