Disturbances of blood glucose control around the time of menstruation are often reported by women with Type 1 diabetes. To investigate the possibility that such changes may be due to alterations in insulin sensitivity or peripheral blood flow we have studied 9 women with Type 1 diabetes during the follicular and luteal phases of the menstrual cycle. Insulin sensitivity was assessed by the glucose clamp technique with insulin doses of 40 (low dose) and 100 (high dose) mU m-2 min-1. Glucose disposal rates were 2.5 +/- 0.3 (mean +/- SE) mg kg-1 min-1 during the follicular phase and 3.2 +/- 0.3 mg kg-1 min-1 in the luteal phase with low dose insulin, and 5.9 +/- 0.4 and 6.4 +/- 0.6 mg kg-1 min-1, respectively, with high dose insulin. These differences were not statistically significant. Forearm blood flow, heart rate, and blood pressure were similar during both phases.