A large number of physiological rhythms are controlled by the central nervous system, including hormone secretion. The potency and toxicity of many drugs also change according to the time when they are administered. There are also some studies of the biological rhythms in pain sensitivity. In the present study, central and peripheral antinociceptive effects of aspirin and dipyrone, commonly used analgesics, were evaluated chronopharmacologically in mice. The central antinociceptive activities of these drugs administered intraperiotoneally (100 mg/kg) were investigated in mice using tail clip, tail flick and hot plate tests, and the peripheral antinociceptive activities of these drugs were investigated using a stretching test with acetic acid when the drugs were administered at 9.00 a.m. or 9.00 p.m. The central nociception was not different between the two groups, while the sensitivity to pain was significantly greater with the acetic acid test (p < 0.05) at night than in the daylight in the control group. Similarly sensitivities to the stretching test were higher at night than in the daytime in aspirin and dipyrone groups. © 2006 VSP.