In order to study the structure of a complex of monomeric actin and myosin head (S1) in the presence of nucleotide, non-polymerizable G-actin was prepared by the reaction of G-actin with m-maleimidobenzoic acid N-hydroxysuccinimide ester (MBS) and diazonium(1H)tetrazole (DHT). Although singly-modified MBS-actin forms a 2:1 complex with S1, doubly-modified DHT/MBS-G-actin forms a 1:1 complex with a dissociation constant of approximately 10(-6) M and can activate Mg-ATPase of S1 by 2-fold. Using a synchrotron X-ray source, we have measured X-ray solution scattering from reversible acto-S1 complexes obtained by mixing rabbit skeletal S1 and DHT/MBS-G-actin in the absence of nucleotide or in the presence of MgADP. The Guinier plots of the scattering intensity showed straight lines indicating no large aggregates and gave the radii of gyration of the complexes to be 49 and 46 A in the absence and presence of ADP, respectively. The analysis of the scattering curves showed that a monomeric actin binds to a tip of S1, and that the complex undergoes substantial rearrangement resulting in a more compact structure when it binds ADP.