Changes in diameter of intact first-year Picea glauca and second-year Pinus banksiana cones were continuously monitored during two growing seasons. Early in their development the cones made intermittent increases in diameter. Cones expanded during the evening and night while vapour pressure deficit of air was decreasing or low, and did not change in diameter during the daytime when vapour pressure deficit was high. In a mid-stage of development, reversible diurnal shrinkage and expansion of cones occurred and these fluctuations were inversely correlated with vapour pressure deficit. Late in the development of P. banksiana cones, many rapid daytime fluctuations in diameter were recorded. Maturing cones of Picea glauca decreased in diameter in a step-like manner. Diurnal shrinkage of maturing cones occurred when the vapour pressure deficit was increasing or high and no appreciable change in diameter occurred when vapour pressure deficit was low.