Oily solutions of various kind of drugs were injected into the hind leg of rats and the intramuscular absorption was investigated by the local clearance method. The intramuscular absorption of drugs having moderate lipid solubility was more rapid than that of the oily solvent itself and proceeded by the apparent first-order process. Apparent absorption rate constants were affected mainly by injection volume and by the physicochemical properties of drugs and oily solvents. The kinetic results are also consistent with a postulate that the drug was mainly absorbed after being transferred from the oily phase to the aqueous phase, and a fairly good correlationship between partition coefficient and the apparent absorption rate constant was obtained. In the case of drugs having extremely high partition coefficient, the absorption rates were very slow, for which the solubility of the drugs in the tissue fluid appeared to be a critical factor.
