Biodistribution and metabolism of [N-methyl-11C]m-hydroxyephedrine ([11C]mHED), an analogue of noradrenaline, were assessed in rats. Pretreatment with desipramine, an uptake blocker, reduced uptake of radioactivity in myocardium but not in lung, liver, kidney, and muscle. Brain uptake was negligible. HPLC showed six radioactive metabolites in plasma and liver but none in myocardium. Co-injection of unlabelled mHED or metaraminol with [11C]mHED demonstrated no difference between the in vivo binding potentials for mHED and metaraminol in myocardium.