The primary purpose of the present study was to investigate empirically the effect of multiple hypothesis testing on error rates in factorial ANOVA designs under a variety of controlled conditions. The per comparison, per experiment, and experimentwise error rates were investigated for three hypothesis testing procedures. The specific conditions manipulated included: (1) the number of factors in the design, (2) the number of levels of each factor, (3) the number of observations per cell, and (4) the population values of each null hypothesis (magnitude of the effect). A Monte Carlo computer simulation procedure was used for generation of the data. Type I and Type II errors were tabulated where appropriate fof the three error rates. (Author)