The effects of exogenous ethanolamine concentrations on ethanolamine uptake and its subsequent incorporation into phosphatidylethanolamine were examined. Hamster hearts were perfused with 0.04-1000 microM labelled ethanolamine. Analysis of radioactivity distribution in ethanolamine-containing metabolites revealed an accumulation of labelled ethanolamine when the heart was perfused with greater than or equal to 0.4 microM labelled ethanolamine. The changes in radioactivity distribution indicated that the phosphorylation of ethanolamine had become rate-limiting in the CDP-ethanolamine pathway when the heart was perfused with greater than or equal to 0.4 microM ethanolamine. Perfusion with different concentrations of ethanolamine did not significantly change the intracellular ethanolamine pool. The accumulation of labelled ethanolamine without a corresponding change in the ethanolamine pool suggests that the newly imported ethanolamine did not equilibrate with the endogenous ethanolamine pool. We postulate that the newly imported ethanolamine was preferentially utilized for phosphatidylethanolamine biosynthesis.