The total blood volume of SPF female rats of Wistar-Imamichi strain, a strain that shows a 4-day estrous cycle at very high rates and is widely used in the study of reproductive biology in Japan, was estimated from blood radioactivity after the intravenous administration of 125I-rat serum albumin. The radioactivity of blood showed a rapid decrease until 10 min after injection and a slow decay thereafter. Radioactivity at time zero was estimated by extrapolation from this slow decay curve, and the blood volume was calculated. From 30 rats weighing 150-300 g we obtained the following equation between blood volume (BLV) and body weight (BW), and a high correlation coefficient (r): BLV = 8,4007 + 0.0430 x BW, r2 = 0.92. The blood volume per body weight decreased linearly with increasing body weight, and we obtained the following equation: BLD/100 g BW = 12.25-0.0174 x BW at r2 = 0.92.