The binding of imipramine (IMI) to platelet membranes was investigated in 13 patients suffering from panic attacks (PA), in 5 patients affected by schizophrenic disorder (S), and in 11 healthy volunteers (V). From 6 volunteers, from 5 patients with panic attacks, and from all the schizophrenic patients, blood samples were collected in the spring, whereas from the others the samples were collected in the autumn. IMI binding was studied according to a protocol provided by the WHO. Binding parameters, the maximum binding capacity (Bmax), and the dissociation constant (Kd) were measured after construction of the Scatchard plot. The differences between V and PA and between V and S were tested by analysis of variance followed by a t-test. Overall and intragroup relationships between Bmax or Kd and diagnosis and season were assessed by a 2-way analysis of variance (ANOVA). Bmax (mean +/- SD) was 947 +/- 269 (V), 742 +/- 160 (PA), and 712 +/- 254 (S) fmol/mg protein. V was different from PA (P less than 0.04) and from S (P less than 0.01). Kd (mean +/- SD) was 1.41 +/- 0.6 (V), 1.15 +/- 0.6 (PA), and 0.79 +/- 0.20 (S) nM. V was different from S only (P less than 0.01). Our results show that panic attacks and schizophrenia decrease the binding capacity of IMI in platelets. In addition, we found a significant difference between patients and controls only for the samples taken in the spring. No statistically significant difference was detectable between the 2 groups in the autumn samples.(ABSTRACT TRUNCATED AT 250 WORDS)