Comparisons were made of 3 micrometeorological models which simulated daily rates of the photosynthesis and transpiration in a maize crop. The most complex model, a complete soil-plant-atmosphere model (SPAM), simulated the wind speed, CO2 concentration, atmospheric water vapor content, air and leaf temperatures and radiation distribution in canopies. The model predicted that transpiration rates of maize were highly dependent on air temperature and mildly dependent on the atmospheric water vapor content, but only on wind speed when air temperature was high. Predicted CO2 assimilation rates were strongly dependent on air temperature, as a consequence of the simulated sensitivity of respiration to temperature, but not on water vapor and wind speed. A simplified model assumed that there were no vertical gradients of air temperature, water vapor and CO2 concentrations above and within the maize canopy. This model produced predictions of CO2 assimilation within 12% of the complete model (SPAM), but predictions of transpiration agreed only under conditions that resulted in low transpiration rates; the difference could be as great as 40%, because the moderating effect of the aerodynamic resistance had been ignored. A 3rd model assumed that the maize canopy could be viewed as a "big leaf". This leads to the Penman-Monteith equation for expressing transpiration. Daily transpiration rates calculated by the "big leaf" model agreed is within 10% of the full model (SPAM) simulations, except for conditions producing the lowest transpiration rates. Estimates of CO2 exchange, using the "big leaf" model, are within 5% in all but 3 cases. The "big leaf" model offers substantial potential for estimating the transpiration and CO2 assimilation rates of maize.