Spring oats (Avena sativa L.) are typically harvested in Iowa before mid-summer. Oats are a cool season crop, and their grain yield is frequently reduced by exposure to high temperatures during grain filling. Major emphasis has long been placed on selection for earlier maturity in oat breeding in Iowa because early varieties tend to escape severe heat stress. Often, unavoidable delays in planting or the early occurrence of hot weather in some seasons cause heat damage to be an all-too-frequent problem even with the earliest varieties available. Continued selection for earlier maturity likely would reduce the yield potential of oats by reducing the time available for photosynthesis and assimilation;My objective was to investigate the potential for breeding for increased tolerance to high temperatures and late planting in oats. Four date-of-planting treatments were used to compare the responses of genotypes from two populations of unselected experimental lines and a diverse set of oat varieties as checks;Grain yields were reduced in the three late planted treatments relative to the first or normal planting date treatment. There was a significant genotype-by-treatment interaction for bundle weight, grain yield, harvest index, straw yield, and vegetative growth duration in both populations and for seed number and vegetative growth rate in the population derived from the introgression of Avena sterilis L. germplasm into A. sativa. This population had a greater genetic variance for response to the treatments, but the other population, derived from crosses of lines adapted to Iowa, had a much better mean response. The checks did not vary in their grain yield response, and their mean response was not nearly as good as that of the latter population;The effect of late planting on grain yield was largely attributable to its influence on harvest index--the growth rates and total biological yields of oats in the later treatments were generally high. Lines that possessed good grain yield response to delayed planting generally were poor grain yielders in the first treatment, but a majority of the highest yielding genotypes in the late treatments also had high yields in the first treatment;The general conclusion was that, although there seemed to be genetic variability for tolerance to the stresses associated with late planting, the heritability of this trait was so low that progress from selection would be difficult.