By analysis of the kinetics of its stimulation by cytochalasins, it has been established that monomeric actin catalyzes the hydrolysis of ATP. This necessitates the existence of an actin.ADP.Pi species as an intermediate that will be present under polymerizing conditions and might be involved in the polymerization reaction. Covalently crosslinked actin dimer has been prepared and analyzed. Its spectral properties suggest it may be in the conformation of F-actin. The dimer binds two moles of nucleotide, ATP more strongly than ADP, and they exchange at rates intermediate between the exchange rates of G-actin and F-actin. One site exchanges more rapidly that the other; this site contains about 15% bound ADP.Pi and 85% bount ATP. It is possible that the dimer is a model of the ends of actin filaments. Profilin has been shown to inhibit the ATPase acitivity of monomeric actin as well as the ATPase activity that accompanies actin polymerization. The K(D) for the 1:1 complex of actin and profilin has been found to be 5-10 X 10 to the minum 6M. Profilin inhibits elongation as well as nucleation and can depolymerize F-actin as well as inhibit the polymerization of G-actin.