The release of aspirin from certain suppository bases, mainly oleaginous and water sol., were studied in-​vitro by using pH 8 phosphate buffer as the releasing medium.  In this study, infant-​size suppositories contg. 100 mg aspirin were prepd. and tested by using diffusion cell of 2 compartments sepd. by semipermeable cellophane membrane.  After 5 h, 40.6​% aspirin was released from Witepsol H15 compared to 24.9​% for Suppocire AM.  While a base composed of 50​% from each of the above formulation showed the lowest release (18.6​%)​.  Addn. of 5​% Tween 80 enhanced the total aspirin release.  Aspirin release from a water-​sol. base was 31.7​% after 5 h.  Salicylic acid resulting from the hydrolysis of aspirin in the diffusion medium was detd.  The percent salicylic acid diffused to the total aspirin ranged from 43.7 to 72.6​%.  Also, the addn. of Tween 80 decreased the salicylic acid released from 59.6 to 43.7​%.  The results obtained were compared with a com. suppository contg. 130 mg aspirin.  A direct relationship was found between the release rate consts. and the dialysis rate consts.