Six trace elements were monitored in neural tissue homogenates from White Leghorn hens orally dosed with tri-o-tolyl phosphate (TOTP) or tri-m-tolyl phosphate (TMTP) (200 mg/kg). Treated birds were monitored daily for development of delayed neurotoxicity, and concentrations of calcium, copper, iron, magnesium, manganese, and zinc were measured with atomic absorption spectroscopy at the time of maximal locomotor impairment (27-35 days postdosing). TOTP-treated birds manifested motor deficit by 15 days postdosing, while hens administered TMTP exhibited no signs of delayed neurotoxicity. Total calcium content in the sciatic nerve homogenates from TOTP-dosed hens was significantly less (P < 0.05) at the time of maximal locomotor impairment, while no shifts in the other trace elements were found. Therefore, the ortho isomer of tritolylphosphate elicited symptoms of delayed neurotoxicity in the hen (i.e., organophosphorus ester-induced delayed neurotoxicity or OPIDN) and caused a decrease in total calcium content in the sciatic nerve homogenates, in contrast to effects of the meta isomer. Analysis of neural homogenates at time of maximal locomotor impairment reflected secondary events in the degradative processes, since the initial assault of TOTP happens early after administration. Therefore, at fully developed OPIDN alteration of calcium balance in sciatic nerves is an indicator of axonopathy in a degenerated nerve following chemical injury.