The monoamine transporter of chromaffin granule membrane has two distinct high-affinity binding sites for tetrabenazine and reserpine, which can be assayed by [3H]dihydrotetrabenazine and [3H]reserpine binding, respectively. The functional molecular mass of the components bearing these sites has been investigated by the radiation inactivation technique. The decline of [3H]dihydrotetrabenazine binding activity with increasing radiation doses followed a single exponential, from which a functional molecular mass of 68 kDa was derived for tetrabenazine binding sites. [3H]Reserpine binding activity declined in a more complex way; however, under conditions where high-affinity reserpine binding sites were specifically assayed, the decline was also exponential, corresponding to a functional molecular mass of 37 kDa for these sites. The figures obtained for high-affinity tetrabenazine and reserpine binding sites are consistent with previous values obtained by photoaffinity of tetrabenazine and serotonin binding sites, respectively. It is thus concluded that the monoamine transporter has an oligomeric structure. By the radiation inactivation technique, cytochrome b561 and dopamine beta-hydroxylase have functional molecular masses of 25 and 123 kDa, respectively. The latter value might be attributed to the dimeric form of the enzyme.