Insulin sensitivity in seven joggers was evaluated by a minidose insulin infusion test. The results were compared to those of 22 healthy nonjoggers with a mean body weight within ± 115% of ideal body weight. The mean increase over base line of the individual steady-state insulin levels were Δ 25 μU/ml (range 15-42 μU/ml) for controls and Δ 16 μU/ml (range 11.2-23.4 μU/ml) for the joggers. The smaller incremental rise in insulin concentration most likely is due to an increased insulin turnover. The declines in glucose concentrations from 10 min after the infusion began to a nadir were similar in controls (28 ± 2.4 mg/dl) and in the joggers (33.7 ± 3.6 mg/dl). However, the insulin sensitivity index, defined as the rate of plasma glucose decline divided by the increment rise in insulin, was clearly higher in the joggers (0.098 ± 0.009 vs 0.057 ± 0.005, P < 0.05). We conclude that the joggers as a group are more sensitive to insulin. The simplicity of a mini-dose insulin infusion should be useful in longitudinal studies where the effect of physical training on insulin sensitivity may be of interest.