Background: As we have previously reported (Ped Res 1996,39:310A), serum dexamethasone concentrations may affect neutrophil function. In order to better evaluate the short term effects of dexamethasone in a sepsis model, we sought to determine the pharmacokinetics of this drug in rats. Previous reports of dexamethasone pharmacokinetics in rats are limited and variable. Methods: Six female Wistar rats (300g) underwent cannulation of the internal carotid artery under general anesthesia. After surgery the animals rested for 1 to 3 days. Each animal received either a standard (0.1) or high (10) mg/kg dose of dexamethasone phosphate IM and blood samples were obtained at 0, 30min, 1, 2, 6, 12 and 24 hr. Serum dexamethasone levels were measured by HPLC. To a 100 mcL of plasma, 100 ng of beclomethasone was added as an internal standard. The resulting pellet was extracted three times with 500 mcL of ethyl acetate, and the combine supernatants were evaporated under a stream of dry nitrogen. The residue was dissolved with 100 mcL of ethanol-DSMO (90: 10,v/v) for analysis. Separation was achieved on a 4 micron C18 reverse column, using gradient elution of 30 to 45% acetonitrile in 0.01 M phosphate buffer, pH 2.8. Detection was by absorbance at 240 nm. Standard curves with known amounts of dexamethasone and beclomethasone extracted from animals plasma, not exposed to either agent, were established. The limit of detection of dexamethasone in the system was 1.25 ng/ml and the average recovery of the drug was 42%. Dexamethasone content was determined from standard curves. Results: Standard dose dexamethasone administration appears to follow first order kinetics with peak values of 1.5 mcg/ml occurring 30 minutes, volume of distribution of 648 ml/kg, half-life of 4 hrs, and clearance of 111.2 ml/kg/hr. High dose dexamethasone administration appears to follow zero order kinetics with peak values of 37.8 mcg/ml occurring at 2 hrs, apparent volume of distribution of 264 ml/kg, half-life of 4.8 hrs, and clearance of 38.8 ml/kg/hr. Conclusion: The pharmacokinetics of dexamethasone in rats by IM injection are complex and dependent upon the dose administered. Our data suggest that at a standard dose, the pharmacokinetics parameters resemble previous reports in rats and other species, but at a high dose, changes in volume of distribution and clearance are observed. Speculation: Clinicians may need to more careful evaluate the kinetics of dexamethasone in their patients, especially at high doses.