The breathing rate and PCO2 in end-tidal air have been studied in controls and in patients with endogenous depression (retarded and non-retarded), with neurotic depression, and with schizophrenia. It has been shown that breathing rate goes up and PCO2 down in non-retarded and neurotic depression. Schizophrenia gives more anomalous results. The fact is emphasized that such changes must lead to alterations in pH and other variables. Studies showing some small chemical differences between these clinical entities and control subjects might therefore be explained by these findings.