The involvement of serotonin (5-HT) in the modulation of nociceptive impulse in the spinal cord has been widely studied. However, its activity, considering the nature of noxious stimuli and the type of 5-HT receptors involved, merits to be further elucidated. The present behavioural study was performed to compare the dose-antinociceptive effect relationship of 5-HT in rats, after intrathecal (i.t.) injection (10 microliters/rat), using mechanical (paw pressure), thermal (tail immersion and tail-flick) and chemical (formalin) pain tests. In rats submitted to the paw pressure test, 5-HT was found to possess a dose-dependent antinociceptive activity (0.01, 0.1, 1, 10 and 20 micrograms/rat) when vocalization threshold was assessed as a pain parameter. A peak effect occurred 5 min after the injection and the effect was maintained for 45 min. The lowest active dose was 0.1 microgram (maximum increase in vocalization thresholds, 23 +/- 3%) and a plateau was observed for 10 micrograms and 20 micrograms (maximum increase in vocalization thresholds, 72 +/- 7% and 71 +/- 6%, respectively). When paw withdrawal was assessed, 5-HT induced a weak hyperalgesic effect for the highest dose (60 micrograms), while other doses were ineffective. In the tail-immersion (warmth and cold) and tail-flick tests, different doses (0.01, 0.1, 1, 10, 30, 60 and 100 micrograms/rat) were studied. In the two immersion tests, only the highest doses (60 micrograms and 100 micrograms) significantly increased the withdrawal thresholds from 5 to 45 min after the injection. The maximum effect was observed at 5 min (23 +/- 4% and 21 +/- 6% for 60 micrograms; 27 +/- 3% and 30 +/- 6% for 100 micrograms in the warmth and cold immersion test, respectively). In the tail-flick test, the doses of 30, 60 and 100 micrograms/rat dose-dependently and significantly increased the withdrawal thresholds from 5 to 45 min after the injection, with a maximum effect at 5 min (30 +/- 5% for 30 micrograms; 37 +/- 6% for 60 micrograms; and 45 +/- 4% for 100 micrograms). In the formalin test, 5-HT (10, 25, 50, 75 and 100 micrograms/rat) produced dose-related antinociception. The nociceptive response (licking of the injected paw) was significantly reduced from 25 micrograms (-59 +/- 11%) in the early phase, whereas the lowest active dose in the late phase was 50 micrograms (-46 +/- 17%). For both phases, a total inhibition was obtained with 100 micrograms. It is concluded that the effect of 5-HT on pain tests may differ according to the applied stimulus and the parameter assessed; unspecific effects of 5-HT may modify motor reactions to noxious stimuli. Mechanical test (assessment of vocalization) was the most sensitive to 5-HT. These observations are of importance in order to further study the pharmacological mechanisms involved in 5-HT spinally induced antinociception.