Relativity, if any, between the hygroscopic properties of drugs with moistureproof packaging and atmospheric conditions was examined, and some observations were made on the drugs having critical relative humidity. Results of critical humidity determinations of various drugs by the saturated solution method indicated that the rate of moisture absorption of a drug was alway proportional to the difference between the vapor pressure of the external atmosphere and vapor pressure at the critical humidity of the drug, within a certain range. The same relationship was found to hold for packaged drugs.