We investigated the relationship between delayed emesis caused by cisplatin (CDDP) based chemotherapy and plasma levels of serotonin, 5-hydroxy-indoleacetic acid (5-HIAA), and substance P in two patients with small cell lung cancer. In each of the cases, we used the 5-HT3 receptor antagonist ramosetron every morning on day 1-4 of the chemotherapy. In case 1, plasma levels of serotonin were low, whereas substance P levels increased since 24 hours after the injection of CDDP. The increase in substance P levels paralleled the onset of vomiting. In this case, however, substance P levels decreased and yet vomiting occurred. Similarly, in case 2, plasma levels of serotonin were low, whereas substance P levels increased since 24 hours. The increase in plasma levels of substance P and the onset of vomiting were observed at the same time 2-3 days after cisplatin administration. In this case, however, vomiting was not observed during the 5 days when the substance P was highest. Therefore, we suggested that substance P was closely associated with CDDP-induced delayed emesis, though some chemical mediators other than substance P might also be related to the emesis.