5-(2-Chloro-6-methoxyphenyl)oxazolidine-2,4-dione (49) is the most potent agent selected from a series of 5-substituted oxazolidinediones that were found to cause improvements in glucose tolerance in previously fasted rats and potentiation of insulin release in response to a glucose challenge. These compounds were unique in not producing hypoglycemia below the normal fasting glycemia levels. Substituent effects at positions 2-6 of the phenyl ring were investigated. Optimal positions for substitution were found to be the 2-, 5-, and 6-positions. Variations in the oxazolidinedione ring generally lead to loss of activity. The synthesis and structure-activity relationships of this series are detailed.