The sleep-waking cycle was studied in two strains of mice during continuous exposure over 3 weeks to an ambient temperature of 34 degrees C or 10 degrees C. As compared to the 25 degrees C control period, C57BL/6 mice showed at 34 degrees C a mean increase of 25% in slow-wave sleep (SWS) particularly during the night. Paradoxical sleep (PS) was also increased by 58% during the exposure period particularly during the night and this resulted from an increase in the number of PS episodes. At 10 degrees C SWS was decreased by 9% only during the first week of exposure, whereas PS was reduced by 24% throughout due to a decrease in the number of PS episodes, although their duration increased. The acrophase of the circadian rhythms was not affected by the changes in ambient temperature. In the BALB/c strain at 34 degrees C SWS and PS were increased only during the first week by 13% and 77%, respectively. At 10 degrees C SWS was decreased by 7% throughout and PS was decreased by 36% due to a reduction in the number of PS phases during the light period. On returning to 25 degrees C, a rebound in PS confirmed the PS deprivation in the cold in both strains.