Nausea and vomiting are frequent side-effects of intravenous cancer chemotherapy. How these complications were related to the gastric mucosal function was investigated by measuring the gastric mucosal potential difference (PD). Eight patients with metastatic breast cancer receiving chemotherapy were investigated. The liquid junction-corrected gastric PD and pH were measured with a newly developed microelectrode. The measurements started half an hour before chemotherapy and continued for 4-5 hours. Nausea, vomiting, psychological stress and sleeping episodes were registered. The initial PD values were -34 mV +/- 8 mV (mean +/- SD). During the observation period 6 of 8 patients had one or more episodes of nausea and vomiting. All episodes were preceded by a significant decline in PD. The magnitude of the decline in PD was unrelated to the time-lag between administration of chemotherapy and the occurrence of nausea and vomiting, and there was no correlation between the time for these episodes and the time for the administration of the chemotherapy. One patient had three episodes of severe psychological stress causing a marked decline in PD. The last patient experienced no nausea, vomiting or stress and had no changes in PD. During sleeping periods PD increased significantly.