The nature of the changes in composition and distribution of wheat flour proteins during dough mixing and dough relaxation was investigated. Two hard red spring wheats with contrasting dough mixing strengths were used, viz. Glenlea and Katepwa. Glenlea and Katepwa are cultivars of the Canada Western Extra Strong Red Spring and Canada Western Red Spring wheat classes, respectively. Doughs were mixed for various times (1 min, peak dough development time, 15 min and 30 min) in a GRL-200 mixer, and were subsequently divided into two equal portions. One piece was relaxed for 45 min and the other represented the control for a particular mixing stage and no relaxation time. Proteins were fractionated according to the Orth and O'Brien (1976) method of direct extraction of flour with 0.05 M acetic acid, and the modified Osborne procedure (Chen and Bushuk 1970). Quantitative protein distributions among the resulting fractions were determined. The protein compositions of the different fractions were analyzed by sodiumdodecyl sulphate polyacrylamide gel electrophoresis (pH 8.3) (SDS-PAGE) under reducing and non-reducing conditions, two-step one dimensional SDS-PAGE (pH 8.3), and polyacrylamide gel electrophoresis (pH 3.1) (A-PAGE). (Abstract shortened by UMI.)