Abstract The idea that guanine nucleotides act as chelators of Al 3+ and that Al interrupts the mechanism by which GTP or Gpp(NH)p stimulates the Na efflux in single muscle fibers from the barnacle Balanus nubilus has been tested. As a rule, injection of GTP or Gpp(NH)p into unpoisoned and ouabain-poisoned fibers produces a rise in the 22Na efflux that is usually transitory in nature. Fibers preinjected with GTP show a fall in the Na efflux following the injection of AlCl 3 in an equimolar concentration. If, however, the concentration of Al for injection is halved, then GTP is found to be fully protective. Fibers preinjected with AlCl 3 show little or no response to the injection of GTP. This is also the case with ouabain-poisoned fibers. Ouabain-poisoned fibers preinjected with GTP also show little or no response to the injection of AlCl 3. The stimulatory response to the injection of AlCl 3 into fibers preinjected with 0.5 M GTP is dose-dependent. A graded response is also found when 0.5 M AlCl 3 is injected into fibers preinjected with GTP in varying concentrations. Gpp(NH)p is fully protective against the inhibitory effect of Al injection in unpoisoned fibers. Further, Gpp(NH)p abolishes the biphasic effect of Al injection on the ouabain-insensitive Na efflux. To strengthen the argument that GTP acts as a chelator of Al, a solution mixture of 0.5 M GTP/0.5 M AlCl 3 (pH 1–2) was injected into unpoisoned fibers. This is found to lead to a smaller fall in the resting Na efflux than that obtained by injecting AlCl 3 alone or injecting AlCl 3 after GTP. It is thus quite clear that the barnacle muscle fiber is a useful preparation for studies of this type.