Six male monozygotic (MZ) twin pairs participated in the present study which was undertaken to assess the contribution of heredity to changes in plasma glucose and insulin in response to exercise-training. This group was submitted to a vigorous ergocycle exercise program inducing a surplus in energy expenditure of 1000 kcal/day over habitual energy expenditure during 22 consecutive days. An oral glucose tolerance test (OGTT, 75 g glucose) was performed before and about 16 hours after the experimental period. Fasting plasma glucose as well as its increase over basal level (delta area) during OGTT were not modified by exercise-training (p greater than 0.05), although a marked reduction in insulin was observed, both in the fasting state and during the OGTT (p less than 0.01). To assess the extent to which heredity influenced the response to the treatment, i.e. the genotype-training interaction, within and between MZ twin pairs means of squares were calculated. A significant intrapair resemblance in the response to exercise-training was observed for fasting insulin and delta insulin area/delta glucose area ratio during OGTT, but not for delta insulin area. These results provide some indications about a possible role of the genotype on the sensitivity to reduce plasma insulin in response to exercise-training. However, this hypothesis needs to be substantiated by other experimental data with control over the genotype of subjects.