When memories are retrieved, they can enter a labile state during which the memory may be modified and subsequently restabilized through the process of reconsolidation. However, this does not occur in all situations, and certain "boundary conditions" determine whether a memory will undergo reconsolidation. Naïve male lister hooded rats were trained for 5 days to press a lever in order to retrieve a food reward associated with a pavlovian light stimulus. Three days post-training, animals were injected with either MK-801 (0.1 mgkg(-1); i.p.) or saline vehicle, 30 min before they were placed back into the training context for a retrieval session. Lever pressing was reinforced only by the light stimulus and was restricted to either 10, 30 or 50 presentations of the light conditioned stimulus. After 48 h, animals were again returned to the boxes and light-reinforced lever-pressing activity was recorded. MK-801-treated animals in the 10CS group significantly reduced lever pressing at test, compared to saline controls. In contrast, MK-801-treated rats in the 50CS group demonstrated a significant increase. There was no effect of MK-801 in the 30CS group. Additionally, there were no effects of MK-801 in an analogous, pure instrumental, setting when the cue lights were omitted. The opposing effects of MK-801 under different parametric conditions likely reflect impairments of appetitive pavlovian memory reconsolidation and extinction, respectively. These results demonstrate a competition between reconsolidation and extinction. However, there are also conditions under which MK-801 fails to impair either process.