PLATELETS are aggregated in vivo and in vitro by ADP1,2. Several agents such as thrombin, collagen and certain fatty acids cause aggregation by releasing ADP3. We reported that ADP, 5-hydroxytryptamine and adrenaline do not bring about aggregation of nucleated thrombocytes of chicken (Gallus domesticus), turtle (Pseudemys scripta elegans), alligator (Alligator mississippensis) and smooth dogfish (Mustelus canis), whereas thrombin does4,5. Thrombin seems to aggregate platelets by releasing ADP, and so we wondered whether ADP has any role in the aggregation of turtle thrombocytes by thrombin. We were also interested in the ability of thrombin and collagen to cause release of adenine nucleotides from thrombocytes.