We previously reported extraordinary increases in micronucleated erythrocytes in CD-1 mouse pups exposed to 3'-azido-3'-deoxythymidine (AZT) and dideoxyinosine (ddI; 50/250, 75/375, 150/750 mg/kg/day AZT/ddI) by gavage throughout gestation and lactation, followed by direct pup dosing beginning postnatal day (PND) 4 (Bishop et al. [2004]: Environ Mol Mutagen 43: 3-9). That study was conducted to explore the potential for genetic damage in newborns exposed perinatally to antiretrovirals in order to reduce maternal-infant transmission of HIV-1. Because dramatic increases in frequencies of micronucleated erythrocytes were seen in exposed pups, additional studies were conducted to clarify the relative contribution of each drug to the observed damage. Pregnant CD-1 mice were administered AZT (50, 75, 150 mg/kg/day) or ddI (250, 375, 750 mg/kg/day) by gavage twice daily in equal fractions beginning prior to mating and continuing throughout gestation and lactation. Direct pup dosing (same regimens) began on PND 4. Peripheral blood erythrocytes of male pups were screened for micronuclei on PNDs 1, 4, 8, and 21. Significant increases in micronucleated erythrocytes were observed in pups and dams exposed to AZT at all doses and sampling times. The highest micronucleus levels were observed in pups on PND 8 after the initiation of direct dosing. In contrast, effects seen in pups and dams treated with ddI were minimal. These results demonstrate that AZT, a component of many anti-HIV combination therapies, induces chromosomal damage in perinatally exposed neonatal mice. Comparison of micronucleated cell frequencies induced by AZT alone or in combination with ddI suggests that ddI potentiates AZT-induced chromosomal damage following direct exposure.