The concept of B-esterase buffering against anti-cholinesterase (ChE) insecticide toxicity has been extensively researched in mammalian species. Presumably due to relatively low levels of anti-ChE detoxifying enzyme activity in birds, however, avian species are often more susceptible to the toxic effects of these compounds. We quantified B-esterase buffering of organophosphate (diazinon and methyl parathion) and carbamate (aldicarb and oxamyl) toxicity in nestling European starlings (Sturnus vulgaris). The differential toxicities were studied using mortality, behavioral observation, and inhibitor affinity data. The toxicities of diazinon, methyl parathion, and oxamyl were affected by the removal of butyrylcholinesterase (BChE) using the specific inhibitor tetraisopropylpyrophosphoramide (iso-OMPA). When BChE was absent, aldicarb toxicity was not affected. Theoretically, compounds affected by BChE removal would have a higher affinity for BChE or carboxylesterase (CaE) than acetylcholinesterase (AChE). However, this was only the case for diazoxon, which had a 1,000-fold higher affinity for plasma BChE and CaE than AChE. Methyl paraoxon and aldicarb had a higher affinity for plasma AChE than for BChE or CaE. Oxamyl had similar IC50 values for all three enzymes studied. The generation of IC50 curves for each inhibitor revealed the presence of nonsensitive forms of CaE in both the plasma and brain. Based on the results of this research, there appears to be no strict correlation between mortality data and inhibitor affinities for each esterase that alone can explain the differential toxicities of these compounds.