Activity of L-gulonolactone oxidase (EC 1.1. 3.8) in livers of fetal Rattus norvegicus and Mus musculus was detectable on the 18th day of gestation, increased rapidly to maxima at 15 and 5 days postpartum for the two species, respectively, and thereafter declined to adult levels. L-Gulonolactone oxidase was not detectable in liver or kidney of fetal guinea pigs at any stage of development. Near-term fetal snowshoe hares had higher activities of liver L-gulonolactone oxidase than observed in a large sample of adults. L-Gulonolactone oxidase was detectable in chicken (Gallus gallus) embryos by the sixth day of incubation, increased rapidly in the kidney with no discontinuity at hatching, reached a maximum at about the 35th day from the beginning of incubation, and then declined to adult levels. Barn swallow (Hirundo rustica) embryos appeared to synthesize little if any L-ascorbic acid; nestlings had considerably higher levels of L-gulonolactone oxidase than adults. Tadpoles of three species of frogs had appreciable levels of L-gulonolactone oxidase activity.