A monitoring system for recording very fast variations in blood levels of32P-labeled compounds is described. The flow-cell used is suitable for continuous determinations in anesthetized small laboratory animals (e.g. rats). The technique reported has been developed for hard beta-emitting isotopes. However, changing the counting unit, the described flow-cell can be adapted for gamma-emitting isotopes. With our system, efficiencies of the order of 40% have been obtained for32P in whole blood.