Atrazine is currently the most widely used herbicide in the U.S. Its effects on embryonic development of Xenopus and fathead minnows was analyzed in this study. Embryos were exposed to one of five different exposures: atrazine (0, 20, or 250 μg/L) or ethynyl estradiol (EE2) (1 or 100 μg/L), all containing 0.025% acetone:methanol. Fish and frogs were exposed immediately after fertilization until no later than 180h in development. At all concentrations, atrazine did not significantly affect timing of development, muscle contractions, embryonic length, yolk diameter, or mortality. Atrazine-exposed frogs and fish, however, did exhibit altered morphology. EE2 did not affect timing of development or muscle contractions of frogs and fish. EE2-exposed fish did have larger yolk diameters than the others exposed to atrazine, possibly indicating slower uptake of nutrients from the yolk sac. However, EE2-exposed frogs had no significantly altered timing of development, muscle contractions, embryonic length, mortality, or morphology. There was also no effect of atrazine or EE2 on cartilage development of frogs. Though few effects were seen this early in development, other studies do demonstrate the effects of atrazine later in development.