Outbred female rats from the age of 1 month were kept at rooms with 12 h light: 12 h dark (LD), 24 h light (LL) and 24 h dark (DD) regimens and 2 weeks later were exposed to 3 weekly i.v. injections of NMU at the dose of 50 mg/kg and starting in 2 days after first NMU administration-to variable or static magnetic fields (VMF and SMF). Exposure to LL regimen significantly promoted whereas the exposure to DD regimen significantly inhibited NMU-induced mammary carcinogenesis. RIA of serum samples from rats maintained under different light regimens has shown that exposure to LL followed by decrease of melatonin level and increase of prolactin level at all of modifying factors (NMU, VMF, SMF). Surgical light deprivation inhibits NMU-indiced mammary carcinogenesis in female rats. The primary role of pineal function in the modifying effects of light/dark regimens and electromagnetic fields on mammary carcinogenesis are suggested.