Gender differences after treatment with streptozotocin (STZ) have been previously reported; however, differences in the glucose response to islet transplantation in STZ-induced diabetes in male and female rats after islet transplantation have not been examined. Male and female Wistar-Furth rats were made diabetic using STZ (55 mg/kg BW) and then given an intraportal islet transplant. Control animals received sham injections and sham transplant surgery; diabetic animals received STZ and sham surgery. In male animals, islet grafts contained 0 (diabetic), 250, 500, 1000, 1500, 2000, and 3000 islets; in female rats, grafts were made up of 0, 500 700, 750, 1000, or 2500 islets. STZ treatment had more dramatic effects on male than female rats. During the diabetic phase, body weights of male rats were significantly reduced compared to those of control male animals; this was not observed among females. Although all STZ-treated animals were hyperglycemic, plasma glucose levels in male diabetic rats were significantly higher than those in females during this phase (29.8 +/- 2.1 vs. 24.6 +/- 0.6 mM). After islet transplantation, body weight gain was positively associated with the number of islets transplanted in male rats (r2 = 0.59; P < 0.01), but not in females (r2 = 0.09; P > 0.8). In both male and female rats, animals that received 1000 islets or more were generally normoglycemic by 3 weeks posttransplant (males, 10.8 +/- 2.2 mM; females, 7.1 +/- 0.2 mM). Approximately 60% of male and female animals that received 500 islets achieved a reduction in plasma glucose levels. Mean plasma glucose levels were 17.2 +/- 2.3 in the females and 22.6 +/- 1.0 mM in males. However, a significantly larger proportion of female 500-islet animals (6 of 16) achieved a plasma glucose level of 9.5 mM or less compared with males receiving 500 islets (2 of 30). Multivariate regression analysis suggests that sex and islet number interact to affect glycemic normalization after islet transplantation. Gender differences appear to influence body weight and plasma glucose responses to islet transplantation. This finding may have particular relevance when a marginal number of functional islets are available.