Gamma-glutamyl transpeptidase (GGTP) activity in rat liver plasma membrane and blood serum was investigated as an analytical marker for studying acute and sub-chronic exposure to DDT and toxaphene. Twentyfour hours after a single oral dose of DDT (200 mg per kg body weight) or toxaphene (110 mg per kg body weight), GGTP activity increased two-fold in liver plasma membranes, followed by a return to normal values 48 h post-exposure. In addition, serum GGTP activity doubled following acute exposure to DDT or toxaphene, and remained elevated for 48 and 96 h , respectively. Following 2 months continuous exposure to eitcer insecticide, serum GGTP activity levels increased to double that of control values. GGTP activity in liver plasma membranes was elevated 2.5-fold following the initial treatment with toxaphene (16.5 mg per kg body weight per day) and the activity remained high throughout the treatment period (120 days). Prolonged exposure with DDT (30 mg per kg body weight per day) did not produce any change in liver GGTP activity. Based on these results, it appears that GGTP activity could be used as a marker for acute DDT and toxaphene exposure.