The focus of the current proposal is to examine the effect of two dynamic simulations on the participants' conceptions of rate of change. Conceptions of rate of change were measured according to Carlson et al.'s (2002) Mental Actions framework and how the participants related the physical simulations to the graphical representations (Heid, et al., 2006). Results indicate that the simulations increased participants' covariational understanding, but did not help the students create a more accurate understanding of rate of change. [For complete proceedings, see ED581294.]