This research is concerned with the determination of the intensity fluctuation spectra of muscle. From the measurement of intensity fluctuation spectra of quasi elastically scattered laser light on muscle cells myosin and actin filaments, we expect to acquire information about, and develop an understanding of, the fluctuating contractile forces that produce structural fluctuations in contractile cells. To achieve these results we plan to use isolated thick and thin filaments from vertebrate and invertebrate muscles as well as the muscles themselves. Studies of the dynamic properties of bilayer membranes are also planned in order to estimate their contribution to the structural dynamics of muscle. Other contractile or motile phenomena as they occur in nerve cells (axoplasmic flow and transmitter release) and in bacteria will also be studied. These studies will provide a basis for understanding the elementary mechanisms of contractility and motility. They will also yield a technology and methodology for studying in situ the functional state of human skeletal and cardiac muscle, and microcirculation by laser light-scattering.