The use of urinary excretion data to evaluate drug absorption was discussed from the theoretical viewpoint and results of studies conducted to determine absorption of aspirin, benzyl penicillin, and sulfaethylthiadiazole were interpreted according to these considerations. It was shown that the absorption of aspirin was apparently rate limited by the time necessary for this drug to dissolve in vivo and that benzyl penicillin absorption appeared to be rate limited by the intrinsic solution rate properties of the salts used. The absortion rate of sulfaethylthiadiazole was calculated by means of excretion rate data and the results obtained shown to be in excellent agreement with calculation of the same quantity made from previously reported blood level data. The calculated absorption rate of this drug was about 1,200 mg./hr., fifteen to thirty minutes after the ingestion of 1-Gm. doses.