The dynamic behaviour of the plasma parameters, including the local electrondensity, electron temperature, plasma potential and their radial gradients, isstudied experimentally during the transitions between low and high confinementmodes (L-H transitions) in the H-1 heliac. In particular, a dynamic phenomenonoccurring near the threshold conditions for the L-H transition and manifestingitself as a quasi-periodic low-frequency modulation of the local and averageplasma parameters is described and discussed. A simple qualitative model forthis phenomenon, referred to as L-H cycles, is suggested.