We have previously shown that constant darkness enhances autoimmunity to type II collagen (CII) and exaggerates development of collagen-induced arthritis (CIA) in DBA/1 mice. This effect was suggested to be mediated via the major hormone of the pineal gland, melatonin, since this hormone (1) is known to be strongly dependent on environmental lighting and (2) has been reported to affect the immune function. The present study was performed in order to clarify if melatonin could account for the previous observation that mice kept in darkness had a more pronounced development of CIA. First, DBA/1 mice kept in constant darkness were analysed for serum melatonin levels. An increase in background levels in comparison to mice kept in a normal dark/light cycle or in constant light was recorded. Then, different groups of mice (kept in constant light in order to minimize endogenous melatonin levels) were immunised with rat CII to induce arthritis and injected with melatonin. Melatonin injections were performed daily (1 mg/kg b.w.) in the afternoons (at 4 p.m.) for 10 days at two different periods: day 1-10 after collagen injection, or at the onset of the disease (day 30-39). Mice injected with melatonin at day 1-10 developed a more severe arthritis while those injected at onset did not differ significantly from the corresponding controls. Our results support the hypothesis that the pineal gland can exaggerate the development of CIA via a high release of melatonin, probably via enhancement of T-cell priming.