Glucose and insulin responses to oral and intravenous glucose (1 g/kg body weight) were measured after consumption of a high fat (HF) or low fat (LF) diet for 3 wk in conscious rats with implanted intravenous and intra-arterial catheters. The HF diet resulted in impaired glucose tolerance and insulin resistance after both oral and intravenous glucose; the effect was more pronounced after oral glucose. In an attempt to understand the basis of the impairment of glucose tolerance after consuming the HF diet, the activity of hepatic glucokinase and the rate of intestinal glucose uptake were also measured. The more severe impairment of glucose tolerance by oral rather than intravenous administration was not explained by an increased rate of intestinal glucose uptake. Indeed, there was a small but significant reduction in the rate of jejunal glucose uptake in the HF rats. However, the greatly reduced activity of hepatic glucokinase in the HF rats was consistent with a reduced capacity for hepatic glucose uptake, which may have contributed significantly to the impaired glucose tolerance. The effects of the HF diet on the insulin response to glucose were much more pronounced after oral rather than intravenous glucose administration. This indicated that the HF diet may have stimulated the enteroinsular axis. However, it is also possible that the particularly high circulating insulin levels, resulting from oral glucose in the HF rats, were a direct response to hyperglycemia, secondary to reduced glucose removal.