Fathead minnow embryos (Pimephales promelas Rafinesque) of 5 different developmental ages (5, 33, 48, 72 and 96 hrs after fertilization) were used as controls and exposed for 2 hrs to a solution of 0.25 μCi ml−1 of3H-Uridine. Another set of embryos (5, 33, 48, 72 and 96 hrs after fertilization) were subjected to the same treatment except that during the one hour immediately preceding the3H-Uridine incubation, the control embryos were placed in water while the experimental embryos were placed in water containing 15 ppm 11.2 LAS. In both cases, radiation counts minute−1 embryo−1 and per milligram of embryo increased over the 4 day developmental period. The embryos with LAS treatment displayed lower radiation counts at all ages as compared to controls, indicating an inhibition of diffusion and uptake of3H-Uridine and/or RNA synthesis. The possible mechanism of LAS is discussed.