Regional cerebral blood flow (rCBF) was measured in awake, freely moving, unstressed rats using the diffusible indicator, [14C]iodoantipyrine (IAP). Rats were prepared with catheters in the abdominal aorta and the right jugular vein and allowed to recover for 5-7 days in a special chamber. The catheters were accessible from the outside of the chamber and permitted cerebral blood flow to be measured without disturbing the rat. The length of the arterial catheter created a time delay and a dispersion of the tracer in blood as it was sampled over time. A correction was made for the catheter length using a numeric solution of the convolution integral. Blood flow in regions composed predominantly of grey matter ranged from 68 to 240 ml/100 g/min with the highest flow in the inferior colliculus. Flows in cortical regions ranged from 103 to 200 ml/100 g/min. Flow in the neural lobe of the pituitary was 454 ml/100 g/min. These studies demonstrate that the errors in measuring arterial tracer concentration when blood is sampled from a long catheter can be corrected. The correction permits rCBF to be measured in freely moving rats.