1-Hydrazinophthalazine [hydralazine (HDZ)] is a hydrazine derivative that is a direct acting vasodilator effective in the treatment of essential hypertension. HDZ is biotransformed by the phase II conjugation enzyme N-acetyltransferase (NAT) forming acetyl HDZ, which spontaneously cyclized to the stable product 3-methyl-s-triazolo- [3,4-alpha]-phthalazine (MTP). Therapeutic use of HDZ has resulted in adverse side effects, specifically a drug-induced systemic lupus erythematosus. Slow acetylators are more likely than rapid acetylators to develop this toxicity. Bacteria expressing different levels of NAT were used to test the hypothesis that acetylation of HDZ decreases its mutagenic potential. The variation in NAT activities was confirmed by incubating bacterial cultures with HDZ, and the formation of MTP was monitored by HPLC. At 1.0 mg/ml HDZ, YG1029 (NAT overexpresser) produced 5.3 times the amount of MTP as TA100 (normal NAT expresser), and this production was linear for 20 hr. In the Salmonella mutagenesis assay, HDZ produced a dose- and strain-dependent increase in the number of revertants observed. Exposure to 4 mg HDZ/plate resulted in 1000 revertants in the overexpressing strain, YG1029, whereas both TA100 and TA100/1,8DNP6, which express normal levels and lack the NAT protein respectively, produced 1600 revertants. Colony hybridization analysis using probes for each of the six possible TA100 reverting mutations was performed to determine the nature of the mutations. The G:C to T:A transversion was the only mutation whose frequency was increased significantly by HDZ. Fifty-four percent of the induced vs. 25% of the spontaneous mutations were C to A transversions.(ABSTRACT TRUNCATED AT 250 WORDS)