Nitrous Oxide and Mammary Carcinoma

Sir,—In view of the favourable effect of nitrous oxide upon myeloid leukaemia (Eastwood et al., 1963) and lymphoma colony forming cells (Bruce, Lin and Bruce, 1970), we have examined the effect of nitrous oxide on mammary adenocarcinoma in mice. After subcutaneous inoculation with tumour M8013 (mammary adenocarcinoma), one group of C5jBl1() mice was placed in one cage in a flow of nitrous oxide 80% and oxygen 20% which was passed for 11 of 13 consecutive days. A second identical group spent 11 of 13 consecutive days exposed to nitrous oxide 50% in oxygen, and a third identical group was kept in air (control group). On the 13th day, the mice were weighed, the tumour diameters were measured and the leucocytes were counted (after bleeding to death). No effect of nitrous oxide 80% or 50% was observed on the growth of the tumours or on the leucocyte count in peripheral blood. However, there was an obvious difference in loss of weight in the mice exposed to nitrous oxide, especially to the larger concentration (table I). Subsequently, we determined