The genotoxic effects of malonaldehyde (MA) and acetaldehyde (AA) were investigated using primary cultures of rat skin fibroblasts. Exposure to MA at 10(-4) to 10(-3) M concentrations resulted in dose-dependent production of micronuclei. MA was approx. 10 times as potent as AA with respect to micronuclei formation. Treatment with 10(-4) and 10(-3) M concentrations of MA for 12 h produced chromosomal aberrations (chromosomal fragments, achromatic lesions and chromatid breaks) in 14 and 34% of metaphases, respectively. At 24 h the corresponding frequencies were 46 and 52%. AA at analogous concentrations produced aberrations in 4 and 14% of metaphases at 12 h, and 20 and 40% at 24 h. Dose-dependent increases in aneuploidy were seen at 10(-4) M and higher concentrations of both aldehydes, with incidences twice as high for MA as for AA.