AbstractActivins βA and βB (encoded by Inhba and Inhbb genes, respectively) are related members of the TGF-β superfamily. Previously, we generated mice with an Inhba knock-in allele (InhbaBK) that directs the expression of activin βB protein in the spatiotemporal pattern of activin βA. These mice were small and had shortened life spans, both influenced by the dose of the hypomorphic InhbaBK allele. To understand the mechanism(s) underlying these abnormalities, we now examine growth plates, liver, and kidney and analyze IGF-I, GH, and major urinary proteins. Our studies show that activins modulate the biological effects of IGF-I without substantial effects on GH, and that activin signaling deficiency also has modest effects on hepatic and renal function. To assess the relative influences of activin βA and activin βB, we produced mice that express activin βB from the InhbaBK allele, and not from its endogenous Inhbb locus. InhbaBK/BK, Inhbb−/− mice have failure of eyelid fusion at birth and demonstrate more severe effects on somatic growth and survival than either of the corresponding single homozygous mutants, showing that somatic growth and life span are supported by both activins βA and βB, although activin βA plays a more substantial role.