Interspecies differences in "B" esterases were evaluated in seven species of wild birds in order to investigate their different susceptibilities to organophosphorus (OP) insecticides. The relationship between "B" esterases and body size and the influence of feeding habits in the evolution of esterase activities were explored. An in vitro inhibition test was carried out with paraoxon and azinphos methyl oxon in order to investigate the role of equilibrium binding parameters (acetylcholinesterase/ organophosphorus compounds affinity) in determining susceptibility of the different species to parathion and azinphos methyl. Esterase activities were different in all species. The smallest bird studied (the sparrow) had the highest brain acetylcholinesterase (AChE) and plasma carboxylesterase (CbE) activities. The largest bird (the cormorant) had the lowest level of both activities. For microsomal CbE, the coot had the highest activity. An inverse correlation was found between brain AChE and plasma CbE activity in relation to body size. Omnivorous species showed the highest brain AChE and plasma CbE activities which were low in species with a specialised diet. The in vitro inhibition test demonstrated the active involvement of equilibrium binding parameters in determining different susceptibilities to these contaminants in birds.