Both plasma MHPG level and red blood cell membrane transports (TM) of L-tyrosine (L-TYR) and L-tryptophan (L-TRY) were evaluated in 29 depressed patients and compared to 16 control subjects. On the basis of MHPG plasmatic levels, we were able to define two subgroups in the depressed population showing a biological homogeneity: in the first one, both MHPG level and TYR TM decreased, while in the second one neither of these indices were disturbed. These biological features were not related to the nosographic subgroups. Bipolar depressed subjects showed a decrease in both MHPG and TYR TM; unipolar depressed subjects exhibited a decrease in both MHPG and TYR TM associated to an increase in TRY TM, whereas no change was found in dysthymic disorders. The time course of MHPG and MT were desynchronized if followed during the antidepressant treatment. At day 7, MHPG level increased significantly, while clinical improvement showed a normalisation of MHPG and TYR TM.