50 university women with consistent and rhythmic menstrual cycles were given Golden's version of the Stroop color-word test to evaluate their concentration accuracy. 50 male undergraduates were also tested as a control sample. The participants took the Stroop test every seven days for four consecutive weeks to allow examination of whether change in the color and word naming activity occurred. A split-plot analysis of the variance of the scores gave a significant difference for the sequence in which the tests were taken, the measures recited during the testing, and the sex of the respondent. When the values for the women were ordered to correspond to stages of their menstrual cycle, the over-all mean accuracy rose slowly to midcycle, then dropped as menses approached. This supports the hypothesis that the women's concentration was at the lowest level during the latter part of the monthly cycle. Women scored significantly higher on the color and word naming task on the Stroop test than men.