The relationship between temperature and the degradation of reconstituted penicillin solutions was evaluated. In all cases storage at temperatures of 25 degrees C or less was sufficient to maintain penicillin stability within acceptable pharmacopoeial limits for 5 days. In addition, the relationship between humidity and dry bulb temperature was examined in order to assess the use of evaporative cooling in lowering the temperature of penicillin solutions in those areas of the country where temperatures commonly exceed 25 degrees C. The results of this study suggest that, with the use of evaporative cooling, penicillin solutions can be maintained in a stable state for a 5-day period without the need for refrigeration.