Several studies have shown that m opioids are neurotoxic and can produce brain damage in rats. This investigation extends these observations at varying doses and two durations for the short-acting remifentanil, which allows more precise control of duration. Fifty physiologically controled rats were randomly assigned to one of five groups. The control group received normal saline solution and the other groups were given remifentanil in doses ranging from 20 to 160 mg/kg/min for 3 h. After 7 days the rats underwent cerebral perfusion fixation, followed by histologic evaluation by a pathologist unaware of the experimental treatment. Twenty additional rats (10 controls) underwent identical procedures but remifentanil was infused for 5 h at 80 mg/kg/min. This dose was chosen based on the results of the first series of experiments indicating the highest dose of remifentanil with sub-maximal incidence of brain damage. EEG activation was evident during opioid infusion and increased with increasing remifentanil dose (P<0.0001). There were no lesions observed in the brain areas in any of the control group rats. Nineteen of forty rats in the 20-160 mg/kg/min groups showed different degrees of brain damage, primarily in limbic system structures and association areas. The severity of brain damage was greater with higher remifentanil dose (P<0.01). 5 h rats also sustained brain damage compared to controls with a greater level of significance (p<0.003), but compared to 3 h rats a trend was suggested but statistical significance was not achieved (p=0.17). Our data indicate that remifentanil produces dose-related EEG activation and limbic system brain damage in rats and worsening of injury with longer infusion.