Understanding the empirical rules that regulate alterations of hippocampal firing fields will enhance our understanding of hippocampal function. The current study sought to extend previous research in this area by examining the effect of substituting a new stimulus for a familiar stimulus in a familiar environment. Hippocampal place cells were recorded while rats chased food pellets scattered onto the floor of a cylindrical apparatus with a white cue card affixed to the apparatus wall. Once a place cell had been recorded in the presence of the white card, the white card was replaced by a black card of the same size and shape. The place cell was then recorded in the presence of the black card. Thirty-six cells were recorded using this procedure. All cells had stable firing fields in the presence of the white card. Both the white and black cards had stimulus control over place cell firing; generally, rotation of either card caused an equal rotation of the firing fields present. When the black card was substituted for the white card, place cells showed time-variant changes in their spatial firing patterns. The change was such that the spatial firing patterns of the majority of place cells were similar in the presence of the white and black cards during initial black card exposures. During subsequent presentations of the black card, the spatial firing patterns associated with the 2 cards became distinct from each other. Once the differentiation of firing patterns had occurred in a given rat, all place cells subsequently recorded from that rat had different firing patterns in the presence of the white and black cards. The findings are discussed relative to sensory-, motor-, attentional-, and learning-related interpretations of hippocampal function. It is argued that the time-variant alteration of place cell firing fields observed following exposure to a novel stimulus in this study reflects an experience-dependent modification of place cell firing patterns.