Rhesus monkeys with bilateral lesions of either dorsal or ventral prefrontal cortex were tested on delayed matching for colours. The animals with dorsal frontal lesions performed as well after the operation as before, whereas those with ventral frontal lesions were severely impaired even at the shortest delay. However, the animals with dorsal frontal lesions failed to learn delayed spatial alternation. The results support the view that the monkeys with dorsal frontal lesions have an impairment in memory for spatial cues or their own movements in space.