The antihyperglycemic potency of n-butanol extract of celery (Apium graveolens) seed was investigated in Streptozotocininduced diabetic rats. Seventy two mature male rats were assigned to four groups, non-diabetic control and three diabetic groups. Diabetes was induced by single injection with STZ (60 mg/kg b.w., i.p.). Rats ≥ 200 mg/dl of blood glucose were used as diabetic. Diabetic groups (D, N, and I) were drenched with drinking water, n-butanol extract (60 mg/kg, b.w.), or injected with insulin (4 IU/animal), respectively, for 15, 25, and 35 days. Blood samples were obtained every 3 days for assessment of plasma glucose concentrations. On day 15, 25, and 35 days, body weight gain was registered and rats were sacrificed. Specimens from pancreases in all groups have been obtained and fixed in formalin (10%) for histopathological and immunohistochemical study. Diabetic rats (D) showed marked increased blood glucose and decreased weight gain, whereas n-butanol extract of celery seed (N) or insulin (I) therapy moderated blood glucose within normal range and enhanced body weight gain. Immunohistochemical results revealed that n-butanol extract of celery seed treatment caused higher scores of positive cells and intensity of staining compared with other diabetic groups (D and I groups). Histological sections of pancreas from non treated diabetic male rats showed necrosis of most cell population and negatively stained for the few remaining beta cells. These changes were time dependent during the studied experimental periods. It can be concluded that drenching of 60 mg/ kg of n-butanol extract of celery (Apium graveolens) seed has hypoglycemic effect and regenerate islets of Langerhans in experimentally-induced diabetic male rats.