We have examined peripheral insulin action in conscious rats chronically fed high fat (60% calories as fat) or high carbohydrate (lab chow) diets using the euglycaemic clamp plus 3 H-2-deoxyglucose technique. A response parameter of individual tissue glucose metabolic rate (the glucose metabolic index, based on tissue deoxyglucose phosphorylation) was used to assess diet effects in eight skeletal muscle types, heart, lung and white and brown adipose tissue. Comparing high fat with high carbohydrate fed rats, basal glucose metabolism was only mildly reduced in skeletal muscle (only diaphragm was significant,p<0.05), but was more substantially reduced in other tissues (e.g. white adipose tissue 61% and heart 33%). No evidence of basal hyperinsulinaemia was found. In contrast, widespread insulin resistance was found during the hyperinsulinaemic clamp (150 mU/l) in high fat fed animals; mean whole body net glucose utilization was 34% lower (p<0.01), and the glucose metabolic index was lower in skeletal muscle (14 to 56%,p< 0.05 in 6 out of 8 muscles), white adipose (27%,p<0.05) and brown adipose tissue (76%,p<0.01). The glucose metabolic index was also lower at maximal insulin levels in muscle and fat, suggesting the major effect of a high fat diet was a loss of insulin responsiveness. White adipose tissue differed from muscle in that incremental responses (maximal insulin minus basal) were not reduced by high fat feeding. The heart showed an effect opposite to other tissues, with an increase in insulin-stimulated glucose metabolism in high fat versus chow fed rats. We conclude that high fat feeding, without a major increase in body weight or basal hyperinsulinaemia, causes widespread but varying degrees of in vivo insulin resistance in peripheral tissues, with major effects in principally oxidative skeletal muscle.