SUMMARY1. Changes induced by adrenalectomy, triamcinolone treatment and deoxycorticosterone (DOC) treatment on the uptake of [3H]‐serotonin into platelets were studiedex vivo, using rat platelets suspended in physiological medium.2. Adrenalectomy caused a decrease in active uptake of serotonin and an associated increase in apparentKm.3. Treatment of the adrenalectomized rats with triamcinolone (0.25 mg/kg), but not DOC (1.0 mg/kg) restored the active uptake of serotonin.4. At triamcinolone dose of 0.5 mg/kg, the serotonin uptake was raised to a level that was significantly higher than normal.5. Kinetic analysis of uptake data showed the apparentKm, to be affected by both triamcinolone and DOC, while the apparent Vmaxof uptake rate was increased by triamcinolone treatmen