Azepatine, a potent alpha adrenergic antagonist, was catalytically reduced with tritium and hydrogen gas to form dihydroazapetine. The pA2 azapetine was 7.9 whereas that of dihydroazapetine was 6.6, corresponding to a 20-fold decrease in potency. 3H-dihydroazapetine is accumulated into three kinetically distinct compartment in the denervated rabbit aorta. Likewise, efflux of the labeled antagonist occurs from three compartments. The rat constant for onset of alpha adrenoreceptor blockade is 3.15 min-1 which is nearly identical to the rate constant for entry of 3H-dihydroazapetine into a rapidly filling compartment (3.86 min-1) possibly representing the extracellular space. These data are consistent with the hypothesis that onset of alpha adrenoreceptor blockade by dihydroazapetine is diffusion limited.