The effects were determined of externally treating mallard (Anas platyrhynchos) eggs with two insecticides (lindane and toxaphene) and two herbicides (paraquat and 2,4,5-T) with formulations and concentrations similar to field applications. Paraquat was the most embryotoxic of the four compounds regardless of the type of vehicle. The LC50 for paraquat was 1.5 lb of active ingredient/ acre in aqueous emulsion and 0.1 lb/acre in the oil vehicle. The other compounds had LC50's that were several orders of magnitude higher. Both paraquat and toxaphene caused some mortality at 1/2 of the field level of application. Paraquat impaired growth and was slightly teratogenic at 1/2 of the field level of application, but required higher concentrations (1.5 to 3 times the field level) to produce brain and visceral defects. Lindane was teratogenic, resulting in multiple defects but only at doses that were greater than five times the field level of application. Toxaphene resulted in defects of the joints at doses close to or exceeding the LC50. The herbicide 2,4,5-T resulted in few toxic effects and relatively few abnormal survivors with gross defects. The overall embryotoxicity with either vehicle was paraquat > lindane > toxaphene > 2,4,5-T on a lb per acre basis. However the potential hazard at exposures of up to five times the field level of application was paraquat > toxaphene; neither lindane nor 2,4,5-T constituted much of a hazard. Both paraquat and lindane were more toxic on a lb-peracre basis when administered in oil vehicle but only paraquat represented a potential hazard at five times the field level of application.