The mutagenicity of dimethylnitrosamine (62759) (DMN) in Neurospora (N)-crassa was studied. Percent survival and forward and reverse mutation rates in the adenine-3 region of N-crassa were determined using vegetative growth mediated, mycelium extract mediated, host mediated, and organ homogenate mediated activation systems. Sprague- Dawley-rats and albino mice were used for the host and organ homogenate mediated activations. DMN was not mutagenic when conidia were treated under any of the four activation systems. In the growth mediated activation system, the reversion frequency increased linearly as DMN concentration increased. Reversion frequency caused by DMN at a concentration of 6.25 micromolar was 300 times the spontaneous reversion frequency. Percent survival decreased somewhat as dose increased. With mycelial extract mediated activation, DMN caused a high reversion frequency and very slight decrease in survival. In the host mediated assay, DMN caused no increases in mutation frequency in conidia recovered from mouse liver, kidney or lung, but the adenine-3 mutation rate was increased in rat liver, kidney, and lung. Mutation frequency was greatest in conidia from livers of treated hosts and was lowest in lungs. With rat and mouse S-9 systems, DMN reversion was highest with the lung and lowest with the liver, and DMN lethality was slight. The authors conclude that the high sensitivity of the growth mediated activation system to DMN mutagenesis indicates its useful for screening environmental promutagens with N-crassa.