The acute toxicity of chlorpyrifos, methylchlorpyrifos, parathion and methylparathion to three age classes of Artemia salina was determined. In general, A. salina 24-h old was less sensitive to these organophosphorous insecticides (OPI) than A. salina 48-h old and A. salina 48-h old was significantly more tolerant than A. salina 72-h old, in contrast, chlorpyrifos was equally toxic to A. salina 48- and 72-h old. There were some differences among the three age classes of A. salina in the relative order of toxicity of OPI tested. The rank order of toxicity to A. salina 48-h old was methylparathion < parathion < methyl-chlorpyrifos < chlorpyrifos, while to A. salina 24- and 72-h old it was methylparathion = parathion < methyl-chlorpyrifos < chlorpyrifos. The protective effect of the cholinergic antagonists atropine, hexamethonium, pirenzepine and 11-(2-((diethyl-amino)methyl)-1-piperidinylacetyl)-5, 11-dihydro-6H-pyrido(2,3-b)-(1,4)-benzodiazepine-6-one (AF-DX 116) and a cholinesterase-reactivating oxime 2-pyridine aldoxime methochloride (2-PAM) on the mortality due to four selected OPI in Artemia salina 24-h old was investigated. The lethal action of OPI tested was completely prevented by pretreatment of Artemia salina 24-h old with 2-PAM (10(-5) M) and atropine (10(-4 )M). However no concentration of hexamethonium, pirenzepine or AF-DX 116 protected 100% of the animals poisoned by LC84 of the OPI selected, maximum protection obtained was 71 to 88%. In contrast, the maximum inhibition of mortality obtained with AF-DX 116 pretreatment was about 55% because this compound was used at concentrations which were non toxic to control Artemia salina. Atropine, hexamethonium, pirenzepine, AF-DX 116 and 2-PAM afforded 50 % protection (IC50) of Artemia salina against mortality by LC84 of the OPI selected at concentrations in the range of 6.62x10(-7)-1.6x10(-6) M, 2. 38x10(-4)-2.05x10(-3)M, 8.91x10(-7)-1.24x10(-6) M, 9.66x10(-8)-1. 34x10(-7 )M, and 1.95x10(-8)-2.73x10(-8 )M, respectively. Pretreatment of atropine plus 2-PAM to determine whether this combination afforded greater inhibition of the lethality induced by four OPI tested than pretreatment with either atropine or 2-PAM alone was investigated. Atropine (10(-5) M) in combination with 2-PAM (10(-7 )M) inhibited completely the acute toxicity of all OPI tested, while the pretreatment with atropine (10(-6) M) plus 2-PAM at the same concentration gave a inhibition of mortality (about 62%) significantly greater than each antagonist alone (about 14 and 46%, respectively).