Magnetic resonance images were made of rabbits at 1.4 tesla for the purpose of measuring the T1 relaxation times of the kidney in vivo with the animal in different states of hydration. In the normally hydrated animal, the outer cortex has a T1 of about 600 msec and there was a steep T1 gradient with an increase to about 1200 msec at the tip of the inner medulla. The relaxation time gradient was not affected by overhydration with intravenous fluid, but dehydration by withholding water for 24 hours decreased the values in the inner medulla. The T1 and T2 relaxation times and total water content were measured in vitro on tissue samples. The values for both T1 and T2 correlated with the regional renal water content.