Four metabolic activation systems (growth mediated, mycelium extract mediated, host mediated, and organ homogenate mediated) were used to study the mutagenic activity of dimethylnitrosamine (DMN) in both forward and reverse mutation systems in the ad-3 (adenine-3) region of Neurospora crassa. DMN was not mutagenic in Neurospora if conidia alone were treated. It was highly mutagenic, however, if conidia were treated with this compound under any of the four activation systems. Quantitative differences in DMN-induced mutation frequencies were observed between in vivo (growth and host mediated) and in vitro (mycelium extract and organ homogenate mediated) activations. The efficiency of the conversion of DMN to a mutagenic metabolite by the organs of rats and mice appeared to be in a reversed order between the host-mediated (liver > kidney > lung) and the in vitro organ homogenate-mediated (lung > kidney > liver) assays. Inductions of reverse mutations in strain N23 indicated that DMN induces base-pair substitution in N. crassa.