Although numerous studies in rats have demonstrated an influence of serotonin (5-HT) on male copulation, no studies have yet to demonstrate whether such a relationship exists in primate species. The present study sought to characterize 5-HT influences on male copulatory behavior of rhesus monkeys by using three different 5-HT agonists: a full 5-HT1A agonist, 8-hydroxy-2-(din-propylamino) tetralin (8-OH-DPAT); a partial 5-HT1A agonist, ipsapirone; and a 5-HT 1C/ID agonist, m-chlorophenylpiperazine (m-CPP). 8-OH-DPAT had a biphasic effect upon ejaculation latency, with low doses (5-10 micrograms/kg) producing a shortening of ejaculation latency (time from initiation of copulation to ejaculation), and the highest dose (100 micrograms/kg) producing a lengthening of ejaculation latency. Intromission frequency (number of intromissions preceding ejaculation) was affected only at 10 micrograms/kg 8-OH-DPAT with monkeys requiring fewer intromissions to ejaculation at this dose. Ipsapirone administration led to a shortening of ejaculation latency at all doses tested (50-800 micrograms/kg), and a reduction in intromission frequency at 200-800 micrograms/kg ipsapirone. Administration of the 5-HT 1C/1D agonist, m-CPP, resulted in an increase in ejaculation latency at 200-400 micrograms/kg m-CPP and mount latency at 400 micrograms/kg m-CPP, but did not affect intromission frequency. In summary, stimulation of 5-HT1A receptors lowered the ejaculatory threshold of the monkeys, while stimulation of 5-HT 1C/1D receptors interfered with copulatory behavior and raised the ejaculatory threshold.(ABSTRACT TRUNCATED AT 250 WORDS)