It is important to determine the longitudinal relaxation time of blood for black blood imaging, as well as for quantifying blood flow by arterial spin labeling (ASL). In this study a circulation system was used to measure blood T₁ under physiological conditions at the new clinical field strength of 3.0T. It was found that 1/T₁ in s⁻¹ was linearly dependent (P < 0.05) on hematocrit (Hct) within a normal range of 0.38–0.46. The relationships were 1/T₁ = (0.52 ± 0.15) · Hct + (0.38 ± 0.06) and 1/T₁ = (0.83 ± 0.07) · Hct + (0.28 ± 0.03) for arterial (oxygenation = 92% ± 7%) and venous blood (69% ± 8%), respectively, which led to estimated T₁ values of 1664 ± 14 ms (arterial) and 1584 ± 5 ms (venous) at a typical human Hct of 0.42. The temperature dependencies of blood T₁ were 22.3 ± 0.6 ms/°C and 19.8 ± 0.8 ms/°C for Hct values of 0.42 and 0.38, respectively. When a head coil transmit/receive setup was used, radiation damping caused a slight reduction (19 ms) of the measured T₁ values. Magn Reson Med 52:679–682, 2004. © 2004 Wiley‐Liss, Inc.