A manometric technique was used to examine the effect of light intensity and CO2 on the relative photosynthetic activity of ten contrasting Lolium genotypes grown in a common environment. Relationships between leaf anatomy and photosynthesis were examined. At a given CO2 concentration, apparent photosynthesis increased with increasing light intensity until ‘light saturation’was reached. The saturating light intensity increased with increasing CO2 concentration. There were significant differences in photosynthetic rate between genotypes where light or CO2 respectively were limiting, but the relative order of the genotypes differed in these two cases. There was also independent variation in response to increasing CO2 to very high concentrations. At approximately 300 ppm CO2 and at light saturation, apparent photosynthesis was negatively correlated with mesophyll cell size. The most efficient leaves under these conditions were smaller and thinner than the less efficient. When light was limiting, apparent photosynthesis was not related to any anatomical feature. There was no indication of a stomatal effect under any conditions. The association between light-saturated photosynthesis and mesophyll cell size was similar to that between cell size and the estimated cell surface: volume ratio.