Members of the actin family of proteins exhibit different biochemical properties when ATP, ADP-P(i), ADP, or no nucleotide is bound. We used molecular dynamics simulations to study the effect of nucleotides on the behavior of actin and actin-related protein 3 (Arp3). In all of the actin simulations, the nucleotide cleft stayed closed, as in most crystal structures. ADP was much more mobile within the cleft than ATP, despite the fact that both nucleotides adopt identical conformations in actin crystal structures. The nucleotide cleft of Arp3 opened in most simulations with ATP, ADP, and no bound nucleotide. Deletion of a C-terminal region of Arp3 that extends beyond the conserved actin sequence reduced the tendency of the Arp3 cleft to open. When the Arp3 cleft opened, we observed multiple instances of partial release of the nucleotide. Cleft opening in Arp3 also allowed us to observe correlated movements of the phosphate clamp, cleft mouth, and barbed-end groove, providing a way for changes in the nucleotide state to be relayed to other parts of Arp3. The DNase binding loop of actin was highly flexible regardless of the nucleotide state. The conformation of Ser14/Thr14 in the P1 loop was sensitive to the presence of the gamma-phosphate, but other changes observed in crystal structures were not correlated with the nucleotide state on nanosecond timescales. The divalent cation occupied three positions in the nucleotide cleft, one of which was not previously observed in actin or Arp2/3 complex structures. In sum, these simulations show that subtle differences in structures of actin family proteins have profound effects on their nucleotide-driven behavior.