Although gamma-butyrolactone (GBL) rapidly converts to gamma-hydroxybutyrate (GHB) in vivo, the lactone gave significantly more prolonged hypnotic effects than GHB when equimolar doses were compared both parenterally and orally in rats. Plasma drug concentrations were higher after GBL administration through both routes, consistent with the observed differences in the pharmacological activity of these two compounds. Oral GBL was absorbed much faster than oral GHB, with the dual effects of decreasing potential first-pass metabolism and elevating plasma drug concentrations to the region where capacity-limited elimination is operative. Parenteral GBL produced a slower initial drug plasma clearance than parenteral GHB. In spite of the rapid metabolism of GBL to GHB, the apparent tissue distribution of these two compounds may be different.