Circannual variations of the bleeding time in the rat

This study was carried out to determine the annual fluctuations of the tail bleeding time in the rat. Bleeding time was determined by transection of the tail of pentobarbital-anesthetized rats. Over a 6-year period, monthly measurement of the tail bleeding time revealed highly reproducible circannual fluctuations. Bleeding times were shortened in the summer (414 +/- 18 s in July), being almost half that observed during the winter span (886 +/- 32 s in January). The difference between these two time points was highly significant (p < 0.001). This study reports for the first time circannual variations of the bleeding time in the rat.