Technical Abstract: Rubisco activase (RCA) constrains the photosynthetic potential of plants at high temperature (heat stress). We hypothesized that endogenous levels of RCA could serve as an important determinant of plant productivity under heat stress conditions. In this study, we investigated the possible relationship between expression levels of RCA and plant yield in eleven European cultivars of winter wheat following prolonged exposure to heat stress. In addition, we also examined the effect of a short-term heat stress on RCA expression in four genotypes of wheat, five genotypes of maize and one genotype of Arabidopsis thaliana. Immunoblots prepared from leaf protein extracts from control plants showed three RCA cross-reacting bands in wheat and two RCA cross-reacting bands in maize and Arabidopsis. The molecular mass of the observed bands was in the range between 40 and 46 kD. Heat stress affected the expression of RCA in a few genotypes of wheat and maize but not in Arabidopsis. In wheat, heat stress slightly modulated the relative amounts of RCA in some cultivars. In maize, heat stress did not seem to affect the existing RCA isoforms (40 and 43 kD) but induced accumulation of a new putative RCA of 45 – 46 kD. The new putative 45 – 46 kD RCA was not seen in a genotype of maize (ZPL 389) that has been shown to display an exceptional sensitivity to heat stress [Ristic and Cass (1993) J Plant Physiol 142: 759-764]. A significant positive linear correlation was found between the expression of wheat 45 – 46 kD RCA and plant productivity under heat stress conditions. The results support the hypothesis that endogenous levels of RCA could play an important role in plant productivity under supra-optimal temperature conditions.