The effect of piperine (1-[5-(1,3-benzodioxol-5-yl)-1-oxo-2,4-pentadienyl]piperidin e), (from Piper nigrum) on the absorptive function of the intestine was studied. In vitro experiments showed that piperine (25-100 microM) significantly stimulated gamma-glutamyl transpeptidase (gamma-GT, EC 2.3.2.2.) activity, enhanced the uptake of radiolabelled L-leucine, L-isoleucine and L-valine, and increased lipid peroxidation in freshly isolated epithelial cells of rat jejunum. The kinetic behaviour of gamma-GT towards substrate and acceptor altered in the presence of piperine. In the presence of benzyl alcohol, an enhanced gamma-GT activity due to piperine was maintained. These results suggested that piperine may interact with the lipid environment to produce effects which lead to increased permeability of the intestinal cells.