The physical state of actin in nerve ending preparations and its relationship to the membranes was studied at the ultrastructural level by negative staining with uranyl acetate before and after treatment with muscle heaving meromyosin (HMM). Actin prepared from synaptosomal or synaptic membrane preparations did not polymerize to fiber formation as readily as striated muscle actin under the same conditions. Treatment of these brain actin preparations with HMM, however, resulted in formation of fibers characteristically decorated with arrowheads which were quite similar to those formed with muscle actin. Treatment of the synaptosomal or synaptic membrane fractions themselves with HMM caused the formation of numerous decorated fibers although fibers were not evident before HMM treatment. This did not occur with the presynaptic vesicle fraction. The studies suggest that at least part of the actin is associated with synaptic membranes and is in a partially polymerized or non-polymerized state; polymerization can be induced by HMM.