We compared the tolerance for low light of selected F1 rice hybrids bred from two cytoplasmic male sterile (CMS) lines, IR62829A and IR58025A, during wet season in 1992 (see table). They were grown in 3-m2 field plots at 15- × 10-cm spacing and fertilized with 60 kg N ha-1. Wooden screens were used to create low light (50% of normal sun-light) from 35 d after transplanting until harvest. Controls were maintained under normal sunlight (about 340 cal cm-2 d-1). The split-plot design with three replications was used with the two light treatments in the main plots and the hybrids in the subplots. The photosynthetic rate (Pn) of the second leaf at vegetative stage (40 d after planting) and the flag leaf at flowering was measured using a portable LI-6000 at near saturated light (above 1000 μE m-2 s-1). The hybrid IR62829A / Vajram showed highest Pn values but marginal (positive) and negative heterosis over restorers in light and shade, respectively. Swarna registered higher heterosis (over restorer) in Pn under both light and shade. The combining ability for Pn seems to be better with IR62829A for Vajram and IR58025A for Swarna.