Nitric oxide is an important bioregulatory agent that may also be an endogenous and exogenous human mutagen. In order to study mutations generated following exposure of a shuttle vector-borne target gene to nitric oxide, mutations were induced in the supF gene of the pSP189 shuttle vector by treatment with nitric oxide in aerobic buffered solution followed by replication of the plasmid in either human Ad293 or Escherichia coli MBM7070 cells. The induced mutation frequency, which increased with nitric oxide dose, was 44-fold greater than the spontaneous background in human cells and > 15-fold greater than background in the bacterial cells when a total of 100 mmol of nitric oxide was oxidatively absorbed/I of pH 7.4 buffer containing the plasmid. The majority of point mutations analysed (61 and 75% for human and E. coli cells respectively) were AT-->GC transitions with GC-->AT transitions (29 and 23%) being the next most prevalent. The overall frequencies of the various point mutations seen in the supF gene were similar in the two cell types, although the distribution of hotspots showed differences. The results are consistent with a mutational mechanism initiated by deamination of DNA bases.