The relative growth rate (RGR) of both Deschampsia flexuosa (L.) Trin. and Festuca ovina L. increased with photosynthetically active radiation (PAR) across the range 4-32 mol m-2 day-1. The responses of both species were similar at all PAR levels and neither showed a saturation of RGR at the highest PAR. Unit leaf rate increased and leaf length ratio decreased with increasing PAR. There was distinct ontogenetic drift in RGR at the highest PAR level and none at the lowest, confirming that the change of RGR was to plant size rather than to time. The differences in natural distribution of the shade-tolerant D. flexuosa and the shade-avoiding F. ovina could not be accounted for by their RGR responses to changes in PAR. The results suggested that the morphological responses of D. flexuosa to variation in PAR in the field may have been underestimated in earlier work.