The mutagenic potential of 1-hydrazino-4-(3,5-dimethyl-1-pyrazolyl) pyridazino [4,5-b]indole(A80a, CAS 135561-93-2), a new antihypertensive agent, was investigated in 3 test systems, according to the current EC Guidelines: Ames test with strains TA1535, TA1537, TA98 and TA100, SCE (sister chromatid exchange) test in V79 cells and micronucleus test in Swiss mice. No indications for a mutagenic potential were detected in bacteria and mice; positive results were obtained in eukaryotic cells. The potential for inducing chromosome aberrations in eukaryotic cells will be studied.