To assess the hypoglycemic activity mechanism of some plants used empirically by the Mexican population as antidiabetics, traditional preparations of Cucurbita ficifolia, Guaiacum coulteri, Lepechinia caulescens, and Psacalium peltatum, water, tolbutamide, and Regular Insulin were administered to three groups of rabbits each: 1. Healthy rabbits with temporary hyperglycemia induced by the subcutaneous administration of glucose. 2. Rabbits with moderate diabetes (fasting glycemia 150-300 mg/dl), induced with alloxan. 3. Rabbits with severe diabetes (fasting glycemia higher than 400 mg/dl), induced with alloxan. The plant preparations had a hypoglycemic effect similar to tolbutamide in healthy and mild diabetic rabbits and had no effect in severely diabetic rabbits. These results suggest that some pancreatic function or the presence of insulin is required for the hypoglycemic activity of these plants.