The unstable behaviour of the nominally stable single crystal samples of f.c.c. metals and alloys subdued to uniaxial straining is due, first of all. to two factors: the deviation of the crystallographic orientation from the nominal [uvw] direction, and imperfection of the inner structure, being the source of 'texture in a single crystal'. It is evident that in deformed single crystals the only parameter, which has an essential effect on plastic anisotropy, is crystallographic orientation. Thus. it is possible to evaluate the variation of crystallographic orientation at the tensile test indirectly, basing on the change of the strain ratio. This statement explains the significance of the strain ratio r vs. strain c function in the analysis of instability of crystallographic orientation in single crystals subdued to the tensile test. In the previous papers the authors have revealed the change of the strain ratio with the strain in several low stacking fault energy copper alloys and silver, and in medium s.f.e. copper. The present paper constitutes. in a way, a supplement to these results. describing the instability of the strain ratio and crystallographic orientation in the high stacking fault energy nickel single crystals. The present paper should be considered as an attempt to explain the different characteristics of instability in nominally stable [ 100], [111] and [110] f.c.c. single crystals.