Three brands of locally available cephalexin capsules were stored in paper bags at 40 degrees C/90% relative humidity. Samples were taken at different time intervals and tested for the effect of storage on disintegration, dissolution and drug content for up to 70 days. Bioavailability testing was carried by administering the capsules to four volunteers before storage and after 70 days of storage. Two brands (A and B) showed increase in disintegration time associated with decrease in the rate of dissolution. The third brand (C) showed similar effect but later on irregular disintegration and dissolution were observed. Such behavior was found to be a result of storage effect on the capsule shell. No effect on drug content was observed for brands A and B but significant loss of activity was observed with some samples of brand (C). Bioavailability testing based upon urine analysis for excreted drug indicated that the rate but not the extent of drug absorption for brands A and B decreased by storage. The results with brand C showed serious fluctuations in absorption after storage for more than 25 days. These findings point out to the importance of proper formulation and protection of solid dosage forms handled in places where high temperature and humidity conditions are likely to exist.