The effects of the volatile oil extracted from the leaves of Rosmarinus officinalis on glucose and insulin levels were investigated in normal rabbits, after the administration of an intraperitoneal glucose tolerance test (GTT). Also, the effects of the volatile oil on fasting plasma glucose levels, were studied in alloxan diabetic rabbits. In normal rabbits, the intramuscular (i.m.) administration of the volatile oil (25 mg/kg) produced 20% (P < 0.05), 27% (P < 0.01) and 55% (P < 0.001) increases in plasma glucose levels, above those of control animals, at the 60, 90 and 120 min intervals, respectively, following the administration of the intraperitoneal (i.p.) glucose test. The same treatment also resulted in a 30% (P < 0.002) decrease in serum insulin level, in comparison with that of control rabbits at the 30 min interval. In alloxan diabetic rabbits, R. officinalis volatile oil increased fasting plasma glucose levels by 17% (P < 0.05) above those of untreated animals 6 h after its administration. These data suggest that the volatile oil of R. officinalis has hyperglycemic and insulin release inhibitory effects in the rabbit.