The fibroblast growth factor receptors (FGFRs) are a family of receptor protein tyrosine kinases that have been shown to mediate a variety of cellular processes including angiogenesis, wound healing, tumorigenesis, and embryonic development. Distinct FGFR mutations in individuals with autosomal dominant disorders of bone growth and development provide a unique opportunity to determine the function of FGFRs during embryonic development. To determine the consequences of these mutations on receptor function, we have made mutations in Xenopus FGFR1 (XFGFR1) and FGFR2 (XFGFR2) that correspond to several of the mutations identified in these dysmorphic syndromes. Analysis of mutant receptor proteins expressed in Xenopus oocytes indicates that all but one have elevated tyrosine kinase activity relative to their wild-type counterparts. Those mutations that give an unpaired cysteine residue in the extracellular domain result in intermolecular disulfide bond formation and covalent receptor dimerization. Microinjection of Xenopus embryos with RNA encoding mutant receptors with elevated tyrosine kinase activity results in ligand-independent induction of mesoderm in animal pole explants. Wild-type XFGFR1 and XFGFR2 do not induce mesoderm when injected at similar doses. Co-injection of RNA encoding a dominant negative FGF receptor, lacking the tyrosine kinase domain, together with RNA encoding various activated FGFRs inhibits mesoderm induction by a receptor activated by a transmembrane domain mutation or extracellular mutations that introduce an unpaired cysteine residue into the extracellular domain but does not inhibit mesoderm induction by receptors bearing a tyrosine kinase domain mutation. These results indicate that different point mutations may activate FGFRs by distinct mechanisms and that ligand-independent FGFR activation may be a feature in common to many skeletal disorders.