To evaluate the relationship between the development of obesity, nervous system activity and, insulin secretion and action, we tested the effect of a 2 month high fat diet in rats (HF rats) on glucose tolerance, glucose-induced insulin secretion (GIIS) and glucose turnover rate, compared to chow-fed rats (C rats). Moreover, we measured pancreatic and hepatic noradrenaline (NA) turnover, as assessment of sympathetic tone, and performed hypothalamic microdialysis to quantify extracellular NA turnover. Baseline plasma triglyceride, free fatty acid, insulin and glucose concentrations were similar in both groups. After 2 days of diet, GIIS was more elevated in HF than in C rats whereas plasma glucose time course was similar. There was a significant increase in basal pancreatic NA level of HF rats and a twofold decrease in the fractional turnover constant (k) was observed, indicating a change in sympathetic tone. In ventromedian hypothalamus of HF rats, the decrease in NA extracellular concentration following a glucose challenge was lower compared to C rats, suggesting changes in overall activity. After 7 days, insulin hypersecretion persisted and glucose intolerance appeared. Two months later there was no longer insulin hypersecretion while glucose intolerance worsened. At all times HF rats also displayed hepatic insulin resistance. On day 2 of HF diet, GIIS returned to normal, after treatment with oxymetazoline, an 2A adrenoreceptor agonist, thus suggesting the involvement of a low sympathetic tone in insulin hypersecretion in response to glucose in HF rats. In conclusion, high fat diet rapidly results in an increased GIIS, at least in part related to a decreased sympathetic tone, which can be the first step of a cascade of events leading to impaired glucose homeostasis.