This experiment was designed to validate a single-day Morris Water Maze procedure used to assess cognitive functioning in rats. Separate groups of randomly assigned rats received either bilateral or unilateral fimbria fornix transections, bilateral or unilateral cortical ablations, or a sham surgical control procedure. Subjects were tested 7 days postoperatively with a modified version of the Morris Water Maze procedure that requires only a single day of training. The results indicated that bilateral fimbria fornix transections severely disrupted acquisition. Unilateral fimbria fornix transections and bilateral and unilateral cortical lesions disrupted acquisition less severely but impaired subsequent test performance. In general, unilateral lesions of both types produced less severe deficits than bilateral lesions. The practical and analytical advantages of the single-day procedure are discussed.