The effect of partial hepatectomy on platelet kinetics has been assessed in the rat. Four-fifths hepatectomy resulted in thrombocytopenia reaching 50 per cent of control platelet counts on the fifth postoperative day. Platelets were found to accumulate in the sinusoids of the liver remnant after partial hepatectomy. Once correction was made for changing blood volume, it was found that during the first two postoperative days 50 per cent of platelets had disappeared from the blood. After this, platelet numbers fell slowly for a further three days following which recovery commenced. Neither 51Chromium labeled platelet survival nor total organ pooling of platelets varied from the normal after four-fifths hepatectomy. Although platelets were shown to accumulate in the liver remnant the numbers involved did not contribute significantly to the thrombocytopenia. The mechanism responsible for the thrombocytopenia was suppression of entry of platelets into the peripheral blood. The observed effect was specific for platelets; erythropoiesis was not suppressed. Megakaryocytes showed a decrease in average diameter and ploidy value, a change seen also after physiologic suppression of platelet production; these findings raise the possibility that the liver is concerned in the elaboration of thrombopoietin. However, as four-fifths hepatectomy results in immediate cessation of entry of platelets into the peripheral circulation, a phase of platelet production in which the principal physiologic regulator has hitherto been considered to have no effect, a second mechanism must be invoked affecting the final step of platelet production.