Cognitive theories of depression propose that depressed mood is caused by depressing cognitions (negative thoughts). In two studies of depressed patients, presentation of external information at a high rate was used to reduce the frequency of negative thoughts, and the subsequent effect on depressed mood examined. Single-case designs were employed in both studies. In the first study, significant reduction in negative thoughts and depressed mood was achieved in each of two selected patients. In the second study, the same procedure produced reduction in negative thoughts which was individually significant for seven of 13 unselected depressed patients, and also significant for the 13 patients as a group. However, this led to little improvement in depressed mood. This appeared to be due to the small extent of thought reduction achieved. Reduction in negative thought frequency correlated significantly with score on the Newcastle diagnosis scale, being greater for neurotic compared to endogenous depression. Mean depressed mood and mean negative thought frequency were significantly positively correlated across subjects.