Genetic improvement of commercial maize hybrids has resulted in higher crop growth rates, which may be attributable to increased rates of photosynthetic carbon assimilation, decreased respiratory carbon losses, or both. The development of more efficient means of estimating maize leaf photosynthesis and respiration in field studies may help to clarify the physiological basis of genetic improvement of maize. The primary objective of the present work was to determine if chlorophyll fluorometry could be used to estimate maize leaf photosynthetic rates in the field. This was accomplished by making simultaneous measurements of gross photosynthetic CO\sb2 assimilation (A\sbG), using leaf gas exchange techniques, and photosynthetic electron transport rates (ETR), using chlorophyll fluorometry. An estimate of leaf absorptance of the incident photosynthetically active photon flux density (PPFD) is required to calculate ETR, and so a method for rapidly estimating leaf absorptance was developed, using a hand-held chlorophyll meter. In experiments with both indoor- and outdoor-grown plants, a linear relationship was found between ETR and A\sbG. This relationship did not vary among three maize hybrids tested, suggesting that measurements of ETR could be used alone to compare A\sbG among different maize genotypes in field experiments. The ratio between ETR and A\sbG was not constant following sudden changes in incident PPFD, and also increased slightly as plants aged. An experimental protocol for assessing maize photosynthesis in the field using chlorophyll fluorometry was developed, based on estimation of ETR at a PPFD of 1200 μmoles m\sp−2 s\sp−1 (ETR\sb1200). Using this technique, it was possible to detect effects of maize genotype, plant age and leaf temperature on leaf photosynthetic activity. Values of ETR\sb1200, multiplied by estimates of canopy absorptance of incident PPFD, were positively correlated with crop growth rates in a single-location field study, confirming that chlorophyll fluorescence techniques could provide information relevant to the agronomic performance of maize. A two-location field study of leaf respiration revealed a negative correlation between leaf respiration rates and total seasonal dry matter accumulation among six commercial maize hybrids. These results suggest that changes in both leaf photosynthetic rates and leaf respiration rates have contributed to improvement of dry matter accumulation in commercial maize hybrids.