Expression of the proto-oncogenes N-myc and c-myc in the human neuroblastoma cell line, IMR32, was examined after reversible inhibition of DNA synthesis by treatment with deferoxamine. After 48 h treatment with 10(-5) M deferoxamine, the number of c-myc expressing cells doubled, while N-myc expressing cells did not change. The expression level of c-myc in each cell also increased. These results suggested that the synthesis of cellular factor(s) involved in the degradation of c-myc RNA or its product was suppressed by reversible inhibition of cell growth. The regulatory mechanisms of expression of N-myc and c-myc may be distinct.