Using the hydrogen clearance technique, we have measured blood flow in the sciatic nerves of healthy, anaesthetized rats at rest, at various arterial blood pressures, and during respiratory acidosis and hypoxia. The majority of hydrogen clearance curves were bi-exponential. The slower component appears to reflect nerve blood flow more accurately than either the fast component or the composite value obtained from both components. Mean nerve blood flow estimated from the slow component of the seventeen bi-exponential hydrogen clearance curves and from the seven mono-exponential curves was 15.8 +/- 1.1 ml min-1 100 g-1 (+/- S.E. of the mean). The mean value of the fast component of the bi-exponential curves was 118 +/- 6 ml min-1 100 g-1 and that obtained from both components was 25.9 +/- 2.6 ml min-1 100 g-1. Sciatic nerve blood flow was measured over a range of arterial blood pressures of 60-160 mmHG. There is a curvilinear relationship between pressure and flow suggesting that the nerve vascular bed responds passively to changes in perfusion pressure. Respiratory acidosis resulted in no significant change in nerve blood flow. The mean flow was 15.5 +/- 1.9 ml min-1 100 g-1. During hypoxia, nerve blood flow decreased to 7.5 +/- 1.4 ml min-1 100 g-1 as a result of a reduction in arterial blood pressure and an increase in vascular resistance. These findings suggest that normal nerve blood flow is high in relation to metabolic activity, especially when compared with the brain.