5-Hydroxytryptamine (5-HT) uptake was studied by using blood platelets from 13 patients with endogenous depression (Hamilton rating scale 33 +/- 7) and 13 healthy volunteers. An improved method with a short incubation time and low substrate concentration was used, and the incubation was performed in Krebs-Henseleit buffer (pH 7.4) at 37 degrees C. A clear difference in 5-HT uptake by blood platelets was noted: The Vmax of the reaction in patients was 39, and in controls 71 pmol per 2 x 10(7) platelets in 5 min. There was no significant difference in the Km. After a 4-week treatment with imipramine, a competitive inhibition of 5-HT uptake with an increased Km was seen; after a similar treatment with amoxapine there was little change in 5-HT uptake. Amoxapine was inferior to imipramine as an inhibitor of 5-HT uptake, also in vitro. There was no difference in clinical recovery in these treatment groups. These results may be of importance so as to understand the potential biological differences between depressed patients and normal persons.