Solubility and dissolution rate of four kinds of sulfonamides (sulfanilamide, sulfisoxazole, sulfadiazine, and sulfisomidine) were determined in various pH solutions. The in vivo absorption patterns of four sulfonamides after administration to rabbits as a suppository with or without buffer reagents were compared by estimating the blood concentration of sulfonamide as a function of time. Higher alkalinity of the buffer solution increases the solubility and dissolution rate. The results of blood concentration studies demonstrated that both the rate and extent of absorption of sulfonamides were considerably enhanced by the rectal administration of the buffered suppository. The in vitro dissolution charactristics of the four sulfonamides at 37°±1 correlated well with the in vivo absorption data ; the logarithm of the absorption rate constant is proprotional to the dissolution rate. From the results obtained, it would be expected that a buffered suppository of sparingly soluble sulfonamides was more effective than that of unbuffered suppository with respect to their absorption from the rectum.