Young animals are resistant to organophosphate-induced delayed neuropathy (OPIDP), although biochemical changes on Neuropathy Target Esterase (NTE) caused by neuropathic organophosphorus esters (OP) are similar to those observed in the sensitive hen. We report here that the resistance of chicks to single doses of neuropathic OPs is not absolute because ataxia was produced in 40-day-old chicks by 2,2-dichlorovinyl dibutyl phosphate (DBDCVP, 5.0 or 10.0 mg/kg s.c.) and by diisopropyl phosphorofluoridate (DFP, 2.0 mg/kg s.c.). However, the clinical picture was different from that usually seen in hens; spasticity and complete recovery being the main features. alpha-Tolyl sulphonyl fluoride (PMSF, 300 mg/kg s.c.) promoted both DBDCVP neuropathy (5.0 or 10.0 mg/kg s.c.) and non-neuropathic doses of DFP (1.5 mg/kg s.c.) or DBDCVP (1.0 mg/kg s.c.). The lowest promoting dose of PMSF given 24 hr after 1.5 mg/kg of DFP was 30 mg/kg. Higher doses had a more severe effect but no further increase of OPIDP severity was obtained with doses ranging from 90 to 300 mg/kg. PMSF (30 mg/kg) protected 40-day-old chicks from subsequent doses of neuropathic OPs even when a promoting dose of PMSF followed. At 60 days of age, chicks' resistance to OPIDP decreased because lower doses of neuropathic OPs became effective and, similarly to hens, PMSF did not fully protect from subsequent promotion. In 40-day-old chicks the threshold of NTE inhibition for OPIDP development was 95-97% (DBDCVP 5.0 mg/kg). When promotion followed initiation, the minimal effective inhibition of NTE for initiation by neuropathic OPs was about 90%. In 36-day-old chicks, PMSF (300 mg/kg) promoted OPIDP when given up to 5 days after DFP (1.5 mg/kg) when residual NTE inhibition in brain and sciatic nerve was about 40%. We conclude that chicks' resistance to OPIDP might reflect either a less effective initiation by phosphorylated NTE or a more efficient repair mechanism or both, and also that promotion is likely to involve a target other than NTE.