Platelet serotonin uptake and platelet serotonin concentrations were measured in 17 patients (5 males, 12 females) with panic attacks and 15 controls (8 males, 7 females). Higher Vmax values were found in the patient group compared with controls (65 +/- 7 vs. 47 +/- 3 pmol/10(8) platelets/min; p less than 0.05) while the affinity constant Km was not significantly different (0.6 +/- 0.1 vs. 0.5 +/- 0.1 microM). Platelet serotonin concentrations were not significantly different between the two groups (38 +/- 4 vs. 41 +/- 4 ng/10(8) platelets). These results confirm our earlier finding of increased serotonin uptake in patients with panic attacks and suggest that platelet serotonin levels are normal.