Lentil grains treated with malathion and stored under laboratory conditions for 12 months formed bound residues. Bioavailability and the effects of lentil-bound residues of malathion in rats were studied. The amount of bound residues in lentils treated with C-14-malathion at 10 ppm and 50 ppm gradually increased to 9.52% and 13.01% of the initially applied doses after 12 months. When rats were fed these C-14-bound residues, radioactivity excreted in urine accounted for 34.49% of the administered dose. In feces, 26.15% of given dose was methanol-extractable while 18.67% was determined as nonextractable. Various tissues including liver, kidney, fat and lungs contained 8.93% while radioactivity in expired air ((CO2)-C-14)) was low (1.51%). The results indicate that lentil-bound malathion residues are highly bioavailable to rats. Analysis of the lentil material containing bound residues indicated that the main compound was malathion.