The effect of gas heating and volume recombination on the constriction in the positive column in glow discharges is analyzed theoretically. Specific results are obtained for helium by solving conservation and diffusion equations for the case where energy balance is dominated by elastic collisions. The degree of constriction is found to be an increasing function of the two parametersRpandIp, whereR, I, andpare the tube radius, tube current, and gas pressure, respectively. In helium the constriction is predicted to set in at a value ofRpof about 750 cm·Torr. This value is much larger than the value (≈30 cm·Torr) at which elastic collisions gain control of the energy balance in helium. Some results calculated for neon gas indicate that volume recombination in the heavier inert gases becomes effective before elastic collisions gain control of the energy balance.