Smad proteins are signal transducers for the members of the transforming growth factor-β (TGF-β) superfamily. Here we show that, in the absence TGF-β stimulation, Smads exist as monomers in vivo. Smad2 and Smad3 form homo-oligomers upon phosphorylation by the constitutively active TGF-β type I receptor, and this oligomerization does not require Smad4. Major portions of Smad4, Smad6 and Smad7 are also present as monomers in vivo. Analysis using a cross-linking reagent suggested that the Smad2 oligomer induced by receptor activation is a trimer. Studies by gel chromatography demonstrated that the Smad2–Smad4 heteromer is not larger than the Smad2 homomer. Moreover, overexpression of Smad4 prevented Smad2 from forming a homo-oligomer. These findings suggest that Smad2 may form a homotrimer, or heterotrimers with Smad4, which are probably composed of two and one, or one and two molecules of Smad2 and Smad4, respectively, depending on the amount of each protein. Gel-mobility shift assay revealed that the Smad3 homomer and Smad3–Smad4 heteromer constitute DNA-binding complexes. Transition of the Smad proteins from monomers to oligomers is thus a critical event in the signal transduction of the TGF-β superfamily members.