Two types of measurements of flux fine structure were performed in simple, plate, fast reactor cells. In the first type, studies were made of the rate at which the fine structure approached equilibrium when test cells were placed in an equivalent homogeneous medium. It was shown that caution must be exercised in applying an infinite plane model to the calculation of heterogeneity effects in plate cells of finite dimensions. In the second type of measurement, detailed activation distributions were measured in a series of cells as a basis for the evaluation of cross sections and calculational models.