Creatinine secretion in the dog has been studied under stop-flow conditions; 43 experiments were carried out in males and 14 in females. The mean free-flow and peak stop-flow creatinine U/P/inulin U/P in the males were 1.03 and 1.42, respectively. In 7 normal females the ratios were 0.95 and 1.14. Secretion could be demonstrated in the female if pretreated with testosterone. The results indicate creatinine secretion with proximal tubule localization. The nature of the transport system was studied and significant inhibition could be demonstrated with 2,4-DNP, organic mercurials, organic acids, and organic bases. The nature of the transport mechanism is discussed.