We examined the relationship between circadian rhythms of body temperature (CTR) and sleep in adult and old female rats. Body temperature was recorded telemetrically for months and sleep for 24 h in a 12:12-h light-dark cycle at 23 degrees C. Some old rats had robust CTRs (old good), and some had unstable or absent CTRs (old unstable). Old unstable rats had lower daily mean body temperatures, smaller daily amplitudes, and a more advanced CTR phase than adult rats and old good rats. Old good rats matched adult rats in all measures. In old good rats, circadian parameters of sleep were normal, whereas in old unstable rats the amplitudes of slow-wave and rapid eye movement sleep were decreased, although the amounts were equivalent. Rhythmic parameters of sleep correlate well with the stability or instability of the CTR, whereas homeostatic regulation of rapid eye movement and slow-wave sleep do not seem to be impaired during aging and are independent of the stability of the CTR.