The aim of this work is to propose methods to test mechanism of synergy of toxic agents in bees. A syaergy between proehloraz, an imidazole fungicide, and deltamethrin, a pyrethroid insecticide, was demonstrated experimentally. The hypothesis is that pmehloraz modifies the penetration or the metabolism of deltamethrin. This hypothesis is tested using a pharmacokinetic box model. A previous experimental work showed that bee instantaneous mortalities were higher, from the lime t 1 to the lime t 2 after spraying, in groups sprayed with deltamethrin at dose D o in the presence of prochloraz (A+P) than in those sprayed with deltamethrin alone at a dose et time as high (etA). We postulate that accrued mortality is proportional to the cumulated internal deltamethrin (1D2). ID 2 of treatment (A+P) had to be greater than ID 2 of treatment (aA) during the period from t 1 to t 2 so that the hypothesis would be consistent with the experimental data. The limit, for which the hypothesis is conceivable, is the ID2(aA ) = ID2(A+I, ) curve. We study, in particular, the asymptotic behaviour of the limit curve when different parameters of the kinetic model tend to 0 or oo. These limits allow to verify quickly and easily whether a mechanism is conceivable or not. As the limits are calculated with algebraic values, the test can be used for other synergies.