Recent studies have shown the existence of reciprocal links between cytokine activity and immunomodulating neurohormones or neuropeptides. In particular, the pineal hormone melatonin (MLT) appears to influence IL-2 activity in cancer. The present study was performed to evaluate which interaction exists between MLT and another important cytokine, tumor necrosis factor-alpha (TNF), which is responsible for both antitumor cytolytic activity and cancer-related cachexia. In a first study, we analyzed MLT circadian rhythm under TNF administration (0.75 mg/day i.v. for 5 days) in 10 metastatic solid tumor patients. In a second study, we evaluated TNF serum levels in 10 metastatic solid tumor patients under therapy with MLT alone (20 mg/day orally in the evening for at least 1 month). In a third study, we have measured concomitantly daily serum levels of MLT and TNF in 30 patients with metastatic solid neoplasms. Nocturnal mean serum concentrations significantly increased in response to TNF injection. MLT therapy induced a significant decline in TNF mean values. Finally, patients with abnormally high MLT diurnal levels showed significantly lower TNF mean concentrations with respect to those with normal levels of the pineal hormone. This study, by showing the stimulatory effect of TNF on MLT secretion and the inhibitory action of MLT on TNF release, would suggest the existence of feed-back mechanisms operating between the pineal gland and TNF released from macrophages in human neoplasms.