Cholinesterase (ChE) and the muscarinic (mAChR) and nicotinic (nAChR) cholinergic receptors were measured in the brains of 86 birds of 20 different species collected in Saskatchewan, Canada during 1991 and 1992. There was a strong correlation between ChE and mAChR, and the ratio of ChE : mAChR was 38% less variable than was ChE alone. In a hypothetical test for minimal acute lethal poisoning of all birds based on a reduction by 50% of the normal ChE measured in each, the use of a single diagnostic threshold value of the ChE : mAChR ratio had a diagnostic sensitivity of 98% and a specificity of 99%, thus making theoretically possible the diagnosis of acute lethal poisoning by anticholinesterase chemicals without the use of reference control values for ChE. Both ChE and mAChR were highly stable during 12 days of postmortem decomposition under several different daily temperature regimes. Substantial changes in these molecules occurred only under conditions of constant high temperature (36 C). Acute lethal poisoning by the organophosphate chlorpyrifos did not affect the density of mAChR. Postmortem decomposition did not appear to be an important confounding factor in the diagnostic interpretation of either ChE or the ChE : mAChR ratio except under hot climatic conditions.