Delayed neurotoxicity induced by leptophos, an organophosphorus insecticide, was intensified in hens when phenylmethylsulfonyl fluoride (PMSF) at dose of 30, 60, and 120 mg/kg body weight was administered at different time intervals (24 hr, 3 days, and 5 days) for each dose of PMSF after the hens were exposed to 30 mg/kg (i.v.) of leptophos. The scores for organophosphorus-induced delayed neuropathy (OPIDN) in all groups treated with 120 mg/kg PMSF were significantly higher than those in the group treated with leptophos only (P<0.05 or P<0.01) and the initial signs of OPIDN appeared 2 or 3 days earlier in the former groups than in the latter group. Further, the greater the PMSF post-treatment dose, the more severe were the signs of OPIDN. These findings indicate that post-treatment with PMSF promotes leptophos-induced OPIDN and reduces the period to OPIDN onset. We also examined the effects of various time intervals between PMSF administration and exposure to leptophos on the development of OPIDN. The OPIDN scores in the two groups of hen treated with PMSF on days 3 and 5 after leptophos exposure were high, especially the score of the 5 days treated group became significantly higher on the 18th and 19th day after leptophos administration than even that of the 24 hr treated group with PMSF (P<0.05). These findings suggest that variations in both the dose of PMSF and the time intervals of PMSF post-treatment may affect the delayed neurotoxicity induced by leptophos. Moreover, these results also indicate that PMSF should not be used for either the treatment or the prevention of OPIDN.