It has been hypothesized that some fornical fibres may instantiate a neuromodulatory reinforcement signal supporting memory acquisition in medial temporal cortical regions. This suggests that fornix transection should impair postoperative new learning more severely than the recall of preoperatively acquired information. Here, postoperative recall of 288 concurrent visuo-spatial discrimination problems acquired preoperatively was unaffected after fornix transection in the macaque, whereas new postoper-ative learning of 72 problems was impaired. This and other recent evidence supports the idea that the main function of the fornix in macaque monkeys is to support new learning about spatio-temporal context.