The effects of temperature variations in the microclimate on two oat varieties were investigated in relation to the growth of this crop as measured by total dry matter and net assimilation rate (N.A.R.). A significant correlation of + 0.691 was obtained between N.A.R. and the difference between day and night temperatures measured on the ground within the plot. Highest N.A.R. values were obtained in material from the fourth (May 29) of the six dates of seeding, when the lowest leaf area was found. Highest yields were not found at this date, as total yield was a function of rate and duration of photosynthesis. A linear decline in grain yield from early to late seeding occurred in 1961 while no differences were observed in 1960. A regression analysis of N.A.R. on the differences between the day and night temperatures revealed that both varieties had a similar response to temperature.