A leaf disc method is described to permit the localized incorporation of (35)S-L-methionine into polypeptides synthesized in individual leaves of maize plants grown in the field. The method of incorporation employs minimal external manipulation of the intact leaf, is simple, repeatable, and may be used at any plant age after leaf emergence. Incorporation (cpm/μg protein) in 12 leaves per plant was compared among three inbred (Oh43, W23, M14) and three F1 hybrid (Oh43/M14, W23/M14, Oh43/W23) genotypes. The incorporation was 40% higher (hybrid versus inbred) in 9 of the 12 leaves studied. Samples from leaf 07 (7th leaf numbered from base of plant) for four inbreds (Oh43, M14, B73, Mol 7) and two pairs of reciprocal F1 hybrids (Oh43/M14, M14/Oh43; B73/Mo17, Mo17/B73) were labelled in situ using the leaf disc method. Each cultivar was sampled at three different ages in each of 1985, 1986, and 1987. High-resolution, two-dimensional isoelectric focusing sodium-dodecyl-sulfate polyacrylamide gel electrophoresis and fluorography were used to display the polypeptides synthesized in the samples. Multivariate methods - Principal Coordinate Analysis, Cluster Analysis, and Standard Deviation Distance - were used to analyze variation and to identify trends in the variation for year, genotype, and age sampled. Our analyses disclose a hierarchy to polypeptide synthesis variation in maize leaves: differences in polypeptide synthesis are greater for year-to-year comparisons than differences due to sample age, which in turn are greater than differences for inbred versus hybrid comparisons.