Rats with lesions of the fimbria-fornix, hippocampus, or hippocampus and amygdala were tested in object discriminations commonly used with monkeys. Two 1-pair object discriminations were learned preoperatively and tested postoperatively. Additional postoperative testing included acquisition of a third 1-pair object discrimination, an 8-pair concurrent object discrimination, and spatial alternation. All lesions impaired performance in the 8-pair object discrimination and in spatial alternation but not in the 1-pair object discriminations. Data from this study and from previous studies indicate that the hippocampus in both rats and monkeys has an important role in the mnemonic processes required for concurrent object discriminations and that variations of the procedure for concurrent object discriminations can be an effective tool for investigating hippocampal function.