AbstractAfter a single administration of3H‐reserpine, 10 μgm, 2.5 μCi, into the yolk of fertilized chicken eggs, gross radioactivity (reserpine plus degradation products) and the unchanged labeled drug, separated by thin layer chromatography, were assayed on the 8th and 19th day of incubation. Nanogram per gram concentrations of unchanged3H‐reserpine were recovered at both times. The3H‐reserpine recovered in the whole egg on the 8th day of incubation accounted for up to 20% of the administered dose. On the 19th day of incubation, brain showed the largest share (28%) of3H‐reserpine as a percentage of gross radioactivity of all organs investigated. Reserpine underwent considerable biodegradation in the developing chick embryo, but the concentrations of unchanged drug recovered in embryonal tissues throughout development were of the same order as those persisting in target organs of rats given a pharmacological dose of