Purpose We aimed to assess influence of estrogen on circadian rhythms of body temperature, heart rates and physical activity, in addition to expression of β-adrenoceptor of the heart. Methods Under general anesthesia with isoflurane, female Wistar rats (7 wk of age, n = 14) were bilaterally ovariectomized, and implanted a telemetry device for measurements of body temperature (Tb), physical activity (Act), and electrocardiogram (ECG) in the abdominal cavity. Two silicon tubes (o.d., 1.57; and length 30 mm) containing 17β-estradiol were subcutaneously placed for one group (n=7, E2(+)), empty tubes for the other (n=7, E2(-)). The tubes were removed 10 days after the surgery. On Days 7 and 21 after the removal, the heart was excised, and the cell membrane of the atrium and ventricle of the heart was prepared for the determination ofβ-adrenoceptor (Western blotting). Tb, Act, and ECG were continuously monitored during the period. Results Before the tube removal, Tb was lower (P<0.05) in the E2 (-) group than that in E2 (+) group, but only at 0:30-1:00 am (37.4 ± 0.1°C and 38.0 ± 0.3°C, respectively), which was not seen on Day 15. Before the tube removal, heart rates (HR) in the E2 (-) group were greater (P<0.05) than those in the E2(+) group (387 ± 14 and 336 ± 13 beats/min (bpm) in the light phase, and 450 ± 11 and 389 ± 11 bpm in the dark phase, respectively). HR in E2(+) group increased (349 ± 11, 375 ± 13, and 377 ± 13 bpm in the light phase; and 422 ± 13, 436 ± 13, and 425 ± 16 bpm in the dark phase on Days 1, 7, and 15, respectively). Difference in HR became unclear on Day 15. The expression of β-adrenoceptor of the heart was changed with plasma estrogen level. Conclusion Estrogen level may affect circadian rhythms of body temperature and heart rates, in which independent mechanism seems to be involved. Moreover, estrogen may influence expression ofβ-adrenoceptor of the heart and HR change after ovariectomy.