Using a 14C-labelled pirimiphos-methyl preparation, the percentage of pirimiphos-methyl residues bound to maize grains after 32 weeks of storage was 13% of the applied dosage, or 38% of total terminal residues. Evidence is presented to show that bound residues of pirimiphos-methyl are bioavailable to the rat: 30%, 2% and approx. 6% of radioactivity were measured in urine expired air, and some organs respectively. A major portion of radioactivity (55%) was eliminated through faeces. Grain-bound pirimiphos-methyl residues (generated after storing whole maize grains with pirimiphos-methyl at concentrations of 10 ppm and 100 ppm) were administered to albino rats for 12 weeks. Body and organ weights, enzyme activities and blood chemistry were tested. There was a significant reduction in body weight gain in female rats. Also a significant reduction in blood cholinesterase activity was observed in both male and female rats fed on grain bound pirimiphos-methyl residues at two dose levels. The white blood cell count increased significantly in male rats fed on the high dose. No significant changes were observed in the other blood chemistry parameters tested. The results indicate that maize-bound pirimiphos-methyl residues can exert adverse biological effects in the rat.