The effects of the volatile oil extracted from Nigella sativa seeds on the levels of glucose and insulin were investigated in the rabbit. The i.p. administration of the volatile oil of N. sativa seeds (50 mg/ kg) to fasting normal and alloxan-diabetic rabbits produced significant hypoglycemic effects. These effects were consistent and time-dependent. In normal animals, 15% and 23% decreases in fasting plasma glucose levels were detected 4 h and 6 h, respectively, after treatment. The same treatment produced 12% and 21% decreases in the fasting glucose levels in diabetic rabbits at the 4 h and the 6 h time intervals, respectively. The administration of the volatile oil was not found to alter basal insulin levels in all animal groups, which might suggest a non-insulin-mediated mechanism of action for the demonstrated hypoglycemic activity. The mode of action of the hypoglycemic effects exhibited by the volatile oil extracted from N. sativa seeds remains to be elucidated.