A potential basis for trial spacing and trial distribution effects was investigated in rats. In Experiment 1, a conditioned stimulus (e.g., CS A) was trained with either massed (e.g., A---->A---->A) or spaced (e.g., A-->A-->A) trials. When trials were massed, brief exposure to the training context (a condition typical of massed training) impaired responding, whereas more extensive exposure to the context during or after training reduced this apparent massed trials deficit. In Experiment 2, different CSs were trained in either a massed (e.g., A-->A-->A--> B-->B-->B-->C-->C-->C) or a distributed (e.g., A-->B-->C-->A-->B-->C, etc.) manner. Trials massed in this sense resulted in impaired responding to the CS, and this impairment was attenuated by posttraining extinction of the context cues. Thus, trial distribution and apparent trial spacing effects are at least in part reversible deficits in performance rather than failures of learning.