A gas chromatographic procedure using methylbicyclophosphorothionate as an internal standard, for the determination of ethylbicyclophosphate in the brain, blood, and liver of small laboratory animals, was elaborated. The compound was extracted from the tissue with acetonitrile, and re-extracted from 2.5% sodium sulfate solution with chloroform. After evaporation of the solvent, the residue was dissolved in acetone. Chromatographic partition was carried out using a glass column (4 m x 0.4 cm) packed with 3% OV-17 on W/H.P. chromosorb at 250°C. Recovery of ethylbicyclophosphate ranged from 65% to 95%. Linear relationship of standard curve within the concentration range from 0.4 to 4.0 µg of ethylbicyclophosphate, was found. Precision for the assay of 2.0 µg samples, was 7.36%.