The influx and efflux of 22Na+ was measured in single muscle fibers of the giant barnacle, Balanus nubilus. These experiments show that intracellular Na+ is divided into two compartments. A rapidly exchanging compartment, identified as myoplasmic free Na+, was evident in both influx and efflux experiments. A very slowly exchanging compartment was evident only in the influx experiments. The size of the rapidly exchanging compartment, expressed as the percentage of total intracellular Na, was estimated from the 22Na+ influx experiments and from intracellular Na+ activity measurements made with Na+-specific glass microelectrodes. Influx measurements showed 58.5% free Na+, and Na+ microelectrode measurements showed 51.6% free Na+. Both the 22Na+ flux analysis and Na+ activity measurements indicate at least two compartments for intracellular Na+.