Changes in the photosynthetic light-response curve during leaf development were determined for the fourth leaf of maize crops sown on 23 April and 10 June. Temperatures were unusually mild during late spring/early summer and neither crop experienced chilling damage. The concept of thermal time was used to take into account the effects of different temperature regimes on developmental stage, thereby enabling photosynthetic light-response data to be combined for both crops to describe the general response. Large variations in the upper asymptote (A(sat)) and convexity (Theta) of the light- response curve occurred during leaf development, but the maximum quantum yield of CO2 assimilation remained relatively constant throughout. Dark respiration rates showed a small but significant decrease with leaf age and generally ranged between 5 and 10% of A(sat). A simple mathematical model was developed to assess the sensitivity of daily leaf photosynthesis (A(L)) to reductions in the A(sat), Theta and the initial slope (Phi) of the light-response curve at different stages of leaf development. On bright sunny days, and at all developmental stages, A(L) was ca. twice as sensitive to reductions in A(sat) than to reductions in Phi and Theta. In overcast conditions, however, all three parameters contributed significantly to reductions in leaf photosynthesis, although the contribution of Phi was greatest during early leaf growth, while older leaves were most sensitive to depressions in A(sat). The implications of these results for modelling the sensitivity of canopy photosynthesis to chill- induced photoinhibition of the light-response curve are discussed.