Nickel single crystals were deformed in. uniaxial tension under active and passive conditions in a l N H2SO4 solution at room temperature. In the presence of the thin passive film formed at 800 mV (SCE), τ0 increases and Stages I and II are suppressed in comparison to crystals deformed in the laboratory atmosphere. In contrast, surface removal prior to and during deformation lowers τ0 and, though only slightly affecting θI, significantly increases the extent of easy glide and Stage II. Abrupt changes in the stress for further plastic deformation occur when crystals are unloaded and held at constant potential in interrupted tests. Surface replicas examined in the electron microscope show that slip lines formed during active dissolution are stronger and more widely spaced than those produced on crystals deformed in air. The spacing and step height on passivated crystals appear to be intermediate between those two extremes. A model of plastic deformation based upon the activation of near-surface dislocation sources is presented in the context of the above observations.