The present work has examined the distribution of the two isoforms of monoamine oxidase (MAO), type MAO-A and MAO-B, in the cortex and medulla of the human and rat kidney. Homogenates of renal cortex and renal medulla were prepared in 67 mmoles l-1 phosphate buffer (pH = 7.2) and MAO activity was determined with [3H]5-hydroxytryptamine ([3H]5HT) and [14C]beta-phenylethylamine ([14C]beta-PEA) as preferential substrates of type A and type B MAO, respectively. Km and Vmax values for the two substrates were also calculated. Both MAO-A and MAO-B are present in the cortex and the medulla of the human and rat kidneys. In the human kidney, MAO-A activity was found to be similar in the cortex (Vmax = 142.70 +/- 45.05 nmoles mg-1 protein h-1) and medulla (Vmax = 133.91 +/- 35.51 nmoles mg-1 protein h-1); MAO-B activity was found to be higher in the cortex (Vmax = 166.19 +/- 19.75 nmoles mg-1 protein h-1) than in the medulla (Vmax = 92.91 +/- 13.22 nmoles mg-1 protein h-1). In the rat kidney, MAO-A was also found to be similar in the cortex (Vmax = 62.35 +/- 1.74 nmoles mg-1 protein h-1) and the medulla (Vmax = 59.42 +/- 0.97 nmoles mg-1 protein h-1) and higher than the activity of MAO-B in the two renal areas (cortex, Vmax = 31.06 +/- 1.09 nmoles mg-1 protein h-1; medulla, Vmax = 14.93 +/- 0.97 nmoles mg-1 protein h-1).(ABSTRACT TRUNCATED AT 250 WORDS)