The estrus cycle affects the circadian body temperature (Tb) and activity rhythms, and progesterone is related to these alterations. However, it is not clear whether estrogen (E2) influences it. The present study examined whether E2 affects the circadian Tb and activity rhythm. Ovariectomized rats were implanted with a silastic plate with or without E2 underneath the dorsal skin (E2(-) and E2(+)), and these along with sham operated rats (SH) were measured for Tb and activity for 2 weeks. The mean Tb was lower, and mean activity was higher in E2(+) than that in E2(-) in the day. In the dark phase, the slope of the relationship between the mean Tb and activity in E2(-) was the greatest. The slope in E2(+) and SH was greater in the light phase than that in the dark phase. The daily peak of Tb and activity was lower in E2(+) than that in E2(-). The appearance of the nadir in Tb was later in E2(+) and SH than that in E2(-). The appearance of the peak in Tb and activity was earlier in E2(+) and SH than that in E2(-). Thus, E2 may modulate the circadian Tb and activity rhythm in female rats.