At an optimal germination temperature loss of viability in wheat seed is reflected in reduced levels of nucleoside triphosphates and nucleotide sugars in the embryo compared to the levels found in high viability embryos during the early hours of imbibition. These differences are magnified on germination at 10° C and are accompanied by a greatly reduced rate of protein synthesis in embryos of low viability compared with the corresponding rate in embryos of high viability. Loss of vigour precedes loss of viability in seed but differences in biochemical parameters between high and medium vigour seed do not become apparent unless germination under stress conditions, e. g. low temperature, occurs. Under these stress conditions, wheat seed of reduced vigour can be distinguished from high vigour seed lots of similar viability by the decreased levels of nucleotides and nucleotide sugars found in embryos from reduced vigour seed lots during the early hours of imbibition and by reduced rates of protein synthesis in these embryos during this same period. Nucleotide analysis is a potentially useful method for rapid assessment of the viability or vigour of a seed lot.