A preliminary study has been carried out on the effect of low level direct current on tumor growth using an experimental tumor model developed from an amelanotic melanoma (T1-4) in the hamster. An inoculum of 2 X 10(6) viable cells was injected s.c. on day 0; on day 7 the tumor-bearing animals were randomly divided into treatment and control groups. On days 7 through 11 inclusive, the treatment group was subjected to electrical current (direct current) at levels from 0.1 to 2.4 mA, for 1 h/day under general anesthesia. Control groups were subjected to the same procedures, with the exception that the electrodes were not connected to the current source. On day 14, the animals were killed and autopsied; their tumors were removed, weighed, and sectioned. Treated tumors decreased in mass (as a percentage of controls) from 89% at 0.1 mA to 2% at 2.4 mA. Increased necrosis of the treated tumors was noted macroscopically and microscopically. On histological examination, it was observed that a thin rim of viable cells remained around the periphery after treatment even at the highest current levels. Similar results were obtained with both stainless steel and platinum-30% iridium electrodes. In separate experiments where the animals were allowed to survive after a treatment period (1 h/day for 5 days at 2.4 mA), the viable cells at the periphery developed into tumors whose mass at 28 days posttreatment averaged only 52% of that of the control tumors. The mechanism of growth reduction is unknown but hyperthermia was shown not to be a factor.