Biochemical biomarkers, such as inhibition of serum butyryl cholinesterase (BCHE) and brain acetyl cholinesterase (AChE), have been useful in studies of interactive effects of pesticides in birds. Examples of interactions due to increased activation or decreased detoxication are reviewed. Studies have shown that hybrid red-legged partridges (Alectoris rufa cross) pretreated with the inducing ergosterol biosynthesis inhibiting (EBI) fungicide, prochloraz, were more sensitive to the toxic effects of the organophosphorous (OP) insecticide, malathion, than controls. A dose of 90 mg/kg prochloraz produced greater inhibition at 1, 4 and 24 h following oral administration of 50 mg/kg malathion, compared to corn oil controls. Pigeons (Columba livia) given 180 or 90 mg/kg prochloraz showed greater inhibition of BCHE activity following malathion administration than did control birds. Starlings (Stumus vulgaris), however, appeared not to be induced by 180 or 300 mg/kg prochloraz, and no difference in BCHE activity following dosing with malathion was apparent in comparison with controls. Other EBIs and OP combinations have been investigated in the partridge. Birds pretreated with prochloraz showed a trend towards greater inhibition of serum BCHE activity at most time points following dosing with the OPs dimethoate and chlorpyriphos. Birds pretreated with the EBI penconazole showed significantly greater inhibition of serum BCHE activity at 1, 4 and 24 h after malathion administration than did controls. The mechanism of increased activation of malathion due to induction of cytochrome P-450 by prochloraz is reviewed. In the case of interactions due to inhibition of detoxication, inhibition of brain AChE activity was a useful biochemical biomarker
