Imipramine receptors were studied in platelets from six healthy young subjects (age between 24 and 38 years), five newborns, and six healthy elderly persons (age between 70 and 81 years). Binding parameters, the maximum binding capacity (Bmax) and the apparent dissociation constant (Kd), were determined by Scatchard's analysis. Level of differences between young subjects and the other groups was determined by Student's t-test. Bmax (mean +/- SD) was 1162 +/- 138 (young persons), 564 +/- 65 (newborn), and 508 +/- 98 (elderly persons) fmol/mg protein. The figure for the young was different from that of the newborn (p less than 0.001) and the elderly (p less than 0.01). Kd (means +/- SD) was 1.78 +/- .69 (young persons), 0.68 +/- 0.13 (newborn), and 0.80 +/- 0.27 nM in the elderly. Kd in the volunteers was different from that in the newborn or the elderly subjects (p less than 0.01). Imipramine receptors in platelets appear to be influenced by development and aging.