Considerable evidence exists suggesting that the so-called neuropathy target esterase (NTE) is involved in the mechanisms responsible for organophosphorus-induced delayed polyneuropathy (OPIDP). Earlier studies in the adult hen, the habitually employed experimental model in OPIDP, have shown that most NTE activity in the brain is centered in particulate fractions, whereas approximately 50% of this activity in the sciatic nerve is encountered in soluble form, with the rest being particulate NTE. In the present work, we have studied the particulate and soluble fractional distribution of paraoxon-resistant phenylvalerate esterase activity (B activity), paraoxon- and mipafox-resistant phenylvalerate esterase activity (C activity), and NTE activity (B-C) according to ultracentrifugation criteria (100,000 g for 1 h). To this effect, two sensitive (adult hen and cat) and two scarcely sensitive (rat and chick) models were used. In all four experimental models, the distribution pattern was qualitatively similar: B activity and total NTE were much greater in brain (900-2,300 nmol/min/g of tissue) than in sciatic nerve (50-100 nmol/min/g of tissue). The proportion of soluble NTE in brain was very low (< 2%), whereas its presence in sciatic nerve was substantial (30-50%). The NTE/B ratio in brain was high for the particulate fraction (> 60%) and low in the soluble fraction (7-30%); in sciatic nerve the ratio was about 50% in both fractions.(ABSTRACT TRUNCATED AT 250 WORDS)