Conducted 6 experiments (n = 229) to determine if mental rotation (MR) can begin before perception finishes, as allowed by continuous flow models but not discrete stage models of information processing. Exps 1-3 found that the effect of shape discriminability on reaction time (RT) was underadditive with the effect of stimulus orientation, suggesting that MR began before shape discrimination had finished and that the 2 processes overlapped in time. The results of Exps 4-6 indicate that MR can overlap with color discriminations as well. In both sets of experiments, however, the amount of underadditivity tended to be much less than predicted by models allowing interference-free overlap. This suggests that MR can overlap with perceptual analysis, contrary to fully discrete models, but that little rotation is carried out during this overlap due to interference between simultaneous discrimination and rotation processes.