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The changes in concentration of heptachlor epoxide (HCE) and oxychlordane (OCD) were studied in the tissues of beef cattle that had previously grazed pasture contaminated with commercial heptachlor. In 25 cows and heifers monitored over 488 days, the mean concentration of HCE decreased from 22.0 to 0.08 mg/kg and of OCD from 5.68 to 0.18 mg/kg. The respective half-lives were estimated to be 66 and 92 days. In a controlled feeding experiment, nine steers were subjected to one of three dietary regimes; a high plane of nutrition for 81 days then a low plane for 140 days; a moderate plane of nutrition for the total period; and a low plane of nutrition for 81 days then a high plane for 140 days. Approximately five months later similar treatments were applied for 101 and 94 days. The rate of decline of HCE and OCD was consistently greatest in animals on the high plane of nutrition and lowest in those on the low plane. Lactation did not appear to have a major effect on the rate of decline in the tissue concentrations of these compounds. No significant (P greater than 0.05) relationships were found between the concentrations of HCE or OCD in the subcutaneous fat and either milk or blood at any stage of the experiment. At slaughter, from 18 to 24 months after the commencement of the experiments, significant correlations (P less than 0.05) were found among concentrations of HCE and OCD in subcutaneous fat, renal fat and fat from the bone marrow.(ABSTRACT TRUNCATED AT 250 WORDS)

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