Introduction In an earlier investigation (Ettlinger and Wegener,31958) three monkeys underwent bilateral ablation of the posterior parietal region, and they were subsequently found to show a defect of reaching which might be interpreted as a degree of "disorientation." At the same time their abilities were unimpaired in tests of tactile discrimination of shape and length. Holmes7(1918) observed a disorder of reaching in patients with bilateral parietal lesions caused by gunshot wounds, and he called attention to the fact that Ferrier had described what appeared to be a similar defect following parietal lesions in the monkey. The present work was designed to examine the defect in reaching following bilateral posterior parietal ablations in the monkey in more detail, to investigate further the possible factors appearing to cause "disorientation," and to examine the animals neurologically and also with a more extensive range of quantitative tests. In the previous