Biopharmaceutical factors of the formulation of injections such as the effect of buffer components, osmotic pressure, and pH of injection solutions were studied using the rat thigh muscle clearance method and in vitro diffusion experiment using muscle slice. 1) It has been shown that the intramuscular absorption of isonicotinamide, a model neutral drug, was hardly affected at all by the buffer components and that of isonicotinic acid, a model anionic drug, was independent for buffer components except potassium ion. 2) Change of the absorption patterns of isonicotinic acid and isonicotinamide due to osmotic pressure of injection solutions was interpreted in terms of a nonspecific, reversible change in the muscle. 3) It was also shown that remarkable reduction of the rate of absorption from acidic solutions was closely related to the irreversible functional change caused by morphological damage.