There is strong evidence that pineal melatonin is involved in controlling neoplastic processes. We have reported that physiological, but not pharmacological or sub physiological, concentrations of melatonin enhance intercellular communication in normal C3H/10T/2 fibroblasts. Gap junctional intercellular communication intervenes in the control of cell proliferation and differentiation, and seems to play a crucial role in suppression of tumor promotion. A number of in vivo studies have shown that extremely low frequency (ELF) magnetic fields (MF) can act as cancer promoters or co-promoters. In vitro, 60 Hz MF have been reported to block melatonin-induced inhibition of cell proliferation in human breast cancer cells. The mechanisms responsible for the observed interactions of MF at the cellular level remain unknown. In the present study melatonin was added to confluent fibroblasts at a concentration of 10(-10) M. Twenty-seven hours later, a fluorescent dye was scrape-loaded into groups of cells and the transfer of the dye to adjacent cells through gap junctions was quantified. Under these conditions melatonin induced a significant increase of dye transfer; this increase was not observed when the cultures were exposed to the MF for 30 min before the previously reported results suggesting that the in vivo oncostatic action of melatonin could be exerted, in part, through modulation of the levels of gap junctional intercellular communication. Also, the data indicate that ELF-MF could counteract the melatonin-induced enhancement of junctional transfer.