Citrated platelet‐rich plasma was prepared from New Zealand white rabbits before, during and after administration of chlorpromazine (2 mg/kg) intramuscularly once daily for 3 to 4 weeks. In these plasmas, the velocity of platelet aggregation by 5‐hydroxytryptamine (5‐HT) added at 1, 3 and 10 μm increased greatly, beginning 3 to 4 days after the start of chlorpromazine injections and lasting for a similar period after they were terminated The increase had two maxima, the first after 6 to 10 days and the second after 17 to 24 days. Chlorpromazine treatment did not affect aggregation by adenosine diphosphate (ADP). The uptake of 5‐HT by rabbit platelets was very fast and linear for less than 10 s. In platelets from untreated rabbits the uptake had a Km of 0.35 ± 0.08 μm and a Vmax of 39.8 ± 6.1 pmol 10−8 platelets 10−1 (n = 5). In platelets from rabbits injected with chlorpromazine (see (2) above) both kinetic constants increased significantly, the Km to 0.88 ± 0.08 μm and the Vmax to 67.8 ± 5.5 pmol. 10−8 platelets. 10−1 s (n = 9).