We examined the diurnal sleep-wake patterns in the adjuvant arthritic rat. In contrast to control rats, arthritic rats lacked a normal diurnal variation in sleep and wakefulness. Thus, arthritic rats exhibited no differences in the mean number or duration of bouts of sleep and episodes of wakefulness between light and dark hours. Arthritic rats also had a marked increase in the fragmentation of their sleep manifested by an increased number of sleep bouts and episodes of wakefulness and a decrease in the duration of episodes of deep sleep recorded both during the time of maximal sleep (08.00-11.00 h) and of maximal wakefulness (20.00-23.00 h). The possibility that the experience of chronic pain causes these marked changes in sleep patterns in the arthritic rat is discussed.