A method has been developed to localize angiotensin-converting enzyme in tissues by in vitro autoradiography. The potent angiotensin-converting enzyme inhibitor, MK351A, was labelled with 125I and shown to exhibit saturable, high affinity, reversible binding to membrane fractions of rat lung and caudate-putamen. The radio-ligand bound with a T 1/2 2-3 min at 20 degrees C and dissociated with a T 1/2 of approximately 5 min on addition of excess unlabelled inhibitor. The potency of a series of converting enzyme inhibitors in displacing the radio-ligand closely paralleled their anticatalytic potency, strongly suggesting that the ligand labels the active site of converting enzyme. In vitro autoradiographic analysis using this system revealed a very high density of converting enzyme in rat lung, small bowel muocosa, adrenal zona glomerulosa, adrenal medulla and brain. In the brain, a discrete and characteristic distribution of the enzyme was observed. A very high density of converting enzyme was found in the choroid plexus, subfornical organ, caudate-putamen, globus pallidus and substantia nigra. The enzyme was also localized in discrete regions of the hypothalamus, thalamus, amygdala, brain stem and cerebellum.