OBJECTIVES This study was performed to search for changes in rat pineal function attributed to age and immunization with Freund's adjuvant. METHODS Young (2 months) and old (18-20 months) Wistar rats were injected s.c. with Freund's adjuvant or its vehicle. Eighteen days later, at the acute phase of arthritis, pineal concentration of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), norepinephrine (NE) and melatonin was measured by high pressure liquid chromatography at 4 different time intervals throughout the nocturnal activity span. RESULTS Old rats had the lowest pineal 5-HT and 5-HIAA content, the decrease in 5-HIAA exceeding that of 5-HT; consequently, old rats had the lowest 5-HIAA/5-HT ratio, an index of pineal 5-HT turnover. Although immunization did not affect globally pineal 5-HT or 5-HIAA levels, significant interactions "immunization x age" and "immunization x time" were found, i.e., immunization augmented pineal 5-HT content at the beginning of the activity span in young rats and at second half of the activity span in young and old rats, and increased pineal 5-HIAA concentration in young rats at the second part of the activity span only. Freund's adjuvant treatment increased pineal 5-HT turnover exclusively in old rats, an effect mainly seen during the second part of the activity span. Old rats exhibited the lowest pineal NE and melatonin levels, immunization further depressing them. CONCLUSION The effect of immunization with Freund's adjuvant on a number of pineal pre- and postsynaptic parameters are age-dependent.