The antinociceptive effect of ricinoleic acid ([R-(Z)]-12-hydroxy-9-octadecenoic acid) in comparison with capsaicin (trans-8-methyl-N-vanillyl-6-nonenamide) has been investigated in several "in vivo" tests. Acute topical application of capsaicin, but not ricinoleic acid, produced by itself an hyperalgesic effect detected as a decrease in paw withdrawal latency in response to a painful (heat) stimulus in mice. Capsaicin, but not ricinoleic acid at any dose tested, showed an irritant effect in the wiping test in guinea pig conjunctiva after local application and in the paw licking test in mice after intradermal injection. Whereas acute application of ricinoleic acid or capsaicin decreased paw withdrawal latency to heat in the presence of a pre-existing inflammation (injection of carrageenan in the mouse paw), the repeated local treatment for 8 days with either compounds markedly increased paw withdrawal latency. In a chronic model of inflammation (complete Freund's adjuvant arthritis in mice), the repeated topical and intradermal treatments with both ricinoleic acid and capsaicin increased paw withdrawal latency to heat, the antinociceptive effect of ricinoleic acid being more persistent than that of capsaicin. Antinociceptive effect of 8 days of treatment with ricinoleic acid and capsaicin was observed in acetic acid-induced writhing in mice, capsaicin-induced foot licking in mice and capsaicin-induced wiping movements in guinea pig conjunctiva. A decrease of substance P tissue levels in the mouse paw was found after repeated treatment with ricinoleic acid. In conclusion, ricinoleic acid seems to be a new antinociceptive agent lacking the pungent and acute hyperalgesic properties of capsaicin.