The hydraulic conductivity, L(p), was determined in single axons of the crayfish, Procambarus clarkii, by injecting a hypertonic sample between two drops of silicone oil and photographing the volume increase of the sample. The method has the advantage of minimizing errors due to hydrostatic pressure differences across the membrane. In resting axons an L(p) of 0.236 x 10(-8) cm/ sec per cm H(2)O was found and similar values were obtained with low external calcium concentration and when the nerve was continuously stimulated at 20-30 impulses/sec. Thus the experiments have failed to demonstrate any change of water permeability in cases in which the ionic conductance is known to change. Some possible implications of this are discussed.