Withdrawal symptoms were studied in 50 volunteer subjects who took secobarbital and 11 who took pentobarbital. The drugs were given by mouth, over periods ranging from 32 to 365 days, at several dosage levels, and 18 of the subjects received the largest daily dose (0.9 to 2.2 Gm. daily) compatible with safe ambulatory management. The symptoms following abrupt withdrawal were insignificant in the patients on minimal dosage but severe in those on maximal dosage; convulsions were seen in 14 instances and delirium in 12. A significant degree of physical dependence can be observed in patients receiving these two drugs, but withdrawal symptoms differ from those that follow withdrawal of opiates and they can be avoided by keeping the dosage below 0.4 Gm. per day.