Objetive: To determine the in vitro and in vivo effects of lidocaine on preimplantation mouse embryo development during prolonged exposure. Study design: In vitro experiments: Two-cell mouse embryos were exposed to lidocaine doses of 0, 10, 100, and 1000 μg/mL for 72 h. In vivo experiments, female mice were exposed to lidocaine doses of 0, 0.3, 1.7, and 3.3 mg/kg body weight on days 1, 2, 3, and 4 of pregnancy. Early development, cell number, mitotic index, and micronuclei frequency were examined. Results: Development to the blastocyst stage was inhibited by all the doses tested in vitro and in vivo. Most of the affected embryos showed arrest and degeneration. The cell number was decreased, but not the mitotic index. Furthermore, clastogenic damage was observed in vitro. Conclusions: Lidocaine adversely affects preimplantation mouse embryo development in vitro and in vivo. Copyright © 1996 Elsevier Science Inc. All rights reserved.