A population of 480 random lines of oats (Avena sativa L.) were grown in low nitrogen (N) (0 kg ha[superscript]-1) and high N (112 kg ha[superscript]-1) environments to assess the effect of N availability in the soil on a plant's ability to partition N between vegetative tissue and grain. The ratio of grain N to total plant N is known as the nitrogen harvest index (NHI). Other traits measured in the low N and high N environments were harvest index (HI), grain yield (GY), straw yield (SY), biological yield (BY), groat protein yield (GTPY), groat protein percentage (GTPP), vegetative protein yield (VPY), straw protein percentage (SPP), and total plant protein yield (TPPY). There was no genotype x environment interaction for NHI. The high N environment caused significant increases in TPPY, VPY, and SPP, and significant reduction in NHI and HI. Low N and high N environments cuased no differential effects on GY, SY, GTPY, and GTPP;The 480 oat lines which had a range from 40 to 74% in NHI when tested in the high N environment were divided into ten strata on the basis of their NHI values. Five lines were selected at random from each stratum to represent the 10 NHI groups. These 50 oat lines plus nine check cultivars were tested in 15 environments with productivity indexes from 0.90 to 3.18 Mg ha[superscript]-1 GY to provide estimates of the association between NHI and adaptability parameters of oat lines to varying levels of soil N. Three yield characteristics, mean across environments, regression response to improving environments, and stability of response, were used to measure adaptability. The groups were significantly variable in grain and straw yield. They were responsive to the improving environments but they were not significantly variable in their responsiveness. Among the yield characteristics, only mean grain yield was significantly correlated with NHI.