It has been shown in studies of turnover in vivo of lactate and several amino acids that the specific activity in blood depends on the site of tracer administration and of sampling. Theoretical and experimental studies have established that to obtain valid estimation of turnover in vivo for these compounds, tracers should be administered into the left heart, ideally the ventricle, and blood sampled from the right heart, ideally the pulmonary artery. Alternately, tracer may be infused into the pulmonary artery and blood sampled from the right ventricle. The insertion of such in-dwelling catheters for turnover studies is difficult in small animals and not justified on ethical grounds in humans. We describe here a method to calculate valid turnover rates when the tracer is administered in a peripheral vein and blood sampled from any peripheral artery. In humans, infusion into a hand vein and sampling from the contralateral heated hand vein can be used. The true turnover rate can then be calculated if the cardiac output is determined. Several noninvasive methods for the determination of cardiac output are available. Thus a correct estimate, avoiding major catheterization, for turnover for many blood-borne compounds is possible.