Diurnal variation of glucose tolerance and insulin action was studied in male Sprague-Dawley rats with a normal or reversed light-dark cycle. A series of experiments conducted was at 12 AM and 12 PM in the two groups. All measurements were separated by a recovery period of at least 3 days and preceded by a 16-hour fast. Glucose tolerance and insulin action were measured by both an oral glucose tolerance test and intraperitoneal insulin tolerance test. Normal light-dark cycle rats had significantly (P < 0.05) greater insulin sensitivity at 12 PM than at 12 AM, whereas reversed light-dark cycle rats had the opposite results (P < 0.05). Rats in the normal light-dark cycle group had a significantly higher growth hormone concentration at 12 AM than at 12 PM, whereas rats in the reversed group had the opposite results. Measurement of insulin-stimulated glucose uptake of isolated adipocytes preincubated with or without 100 ng/ml growth hormone at 37 degrees C for 5 hours revealed that approximately 30% of insulin-stimulated glucose uptake was suppressed when adipocytes were treated with growth hormone. These results indicate that male rats exhibit significant diurnal variation of glucose tolerance and insulin sensitivity, and suggest that the concomitant diurnal variation of growth hormone may have a superimposed and amplifying effects on this variation.