The influence of grain size on the interaction mechanisms between dislocations governing the plastic behaviour of fcc metals at low temperatures was studied by means of thermal activation analysis. For this purpose specimens of single-crystalline, fine-crystalline and sub-microcrystalline nickel, the latter produced by equal-channel angular pressing (ECAP) and pulsed electro-deposition (PED), were deformed in tension at constant strain-rate over(ε{lunate}, ̇)_pl and temperature T in a range 4 K ≤ T ≤ 320 K. Stress relaxation experiments performed during the tensile tests were used to determine the derivative ( ∂ σ / ∂ ln over(ε{lunate}, ̇)_pl )_T as a function of the flow stress σ. In all cases the relationship is found to be linear revealing that the mean dislocation spacing influences both, the athermal and the thermal contribution of the flow stress in the same manner as predicted by the Cottrell-Stokes law. The effect of grain size d on σ can be adequately described through an additive athermal stress contribution σ_d. Moreover, the temperature dependence of the strain-rate sensitivity, m = ( ∂ ln ( σ - σ_d ) / ∂ ln over(ε{lunate}, ̇)_pl )_T, indicates that at very low temperatures the same uniform local single slip mechanism governs the plastic behaviour of nickel single and polycrystals. In this case, m(T) is accurately described by the activation theory assuming the temperature and strain-rate sensitivity of the flow stress σ( over(ε{lunate}, ̇)_pl, T) to be entirely due to the thermally activated interaction of dislocations. Towards higher temperatures m(T) deviates from the predicted single slip behaviour the earlier the smaller the grain size is. The reason probably is the occurrence of additional thermally activated slip processes with higher activation enthalpy, which in PED nickel most likely is the thermally activated interaction of dislocations with grain boundaries.