Developing mammals are more sensitive than adults to acute toxicity from a variety of organophosphorothioate insecticides (OPs), compounds which act in vivo by inhibition of cholinesterase (ChE). Little is known, however, regarding age-related differences in biochemical responses to these toxicants. The time course of ChE inhibition and recovery in whole brain was compared in neonatal (7 days of age) and adult (80-100 days of age) rats after treatment with maximal tolerated doses (MTDs) of either parathion (PS), methyl parathion (MPS) or chloryprifos (CPF). Neonatal rats were more sensitive than adults in all cases (MTDs for PS, MPS and CPF: neonates=2.1, 7.8 and 45 mg/kg, sc; adults=18, 18, and 279 mg/kg, sc, respectively). In general, brain ChE was inhibited to similar degrees (>78%) in both age groups following MTDs of either parathion, methyl parathion or chlorpyrifos but recovered faster in neonates in all cases. The results indicate that neonatal rats are more sensitive to acute toxicity from these compounds and MTD exposures produce extensive ChE inhibition in both age groups. Significant inhibitor-related and age-related differences in the duration of ChE inhibition can ensue, however, following such Op exposures.