The current epidemic of diabetes likely reflects marked changes in environmental factors, although genetic susceptibility plays a powerful role in the occurrence of diabetes in certain populations. We investigated whether long-term sucrose-drinking causes hyperglycaemia in male Wistar-Imamichi littermates (n 32), which are not genetically susceptible to diabetes or obesity. Each litter was divided equivalently into two groups, the sucrose group and the control group. The sucrose group received 300 g/l sucrose water and the control group received regular water until 42 weeks of age. Rats were weighed every 1 or 2 weeks. Oral glucose tolerance tests were performed at 28 and 36 weeks of age. Plasma glucose and insulin concentrations were measured. Body weights were significantly greater in the sucrose group than in the control group in 18-week-old rats (P<0.05), and the difference between the two groups reached 163 g by the end of the study (P<0.01). The 120 min post-load plasma glucose concentration in the sucrose group was 11.4 (SD 2.8) mmol/l in 28-week-old rats and 12.7 (SD 2.2) mmol/l in 36-week-old rats, while that of the control group remained approximately 7.3-7.7 mmol/l. In the sucrose group, the plasma insulin peak occurred 30 min post-load at 28 weeks of age; but the peak disappeared and hyperinsulinaemia was prolonged at 36 weeks of age. In conclusion, long-term sucrose-drinking causes increased body weight and glucose intolerance in normal male rats.