A study of sporadic sodium (Ns) layers observed at S¿o Jos¿ dos Campos (23 ¿S, 46 ¿W) shows that during their occurrence the form of the background sodium layer is different from that which it normally takes when Ns layers are absent. During Ns events, peak sodium in the background layer typically occurs below 90 km, whereas the peak of the average layer observed at our location is around 93 km. The observed change could be caused either by a loss of sodium on the topside of the layer or by a displacement of sodium to lower heights. The consistency of these two mechanisms with our observations, and with the known properties of Ns layers, is examined, but we are unable to determine which of the mechanisms is responsible for the observed phenomenon.¿ 1996 American Geophysical Union