We exposed Swiss ICR mice for 2-hour periods to 1/32, 1/8 and/or 1/2 MAC enflurane, halothane, isoflurane, methoxyflurane, or N2O both in utero during the last 1/2 of pregnancy (4 exposures at 2-day intervals) and after delivery (24 exposures at 2-to-3-day intervals). Anesthetics were delivered in air or in O2. Thus, 1973 mice were exposed and examined after 15 months of life for the development of neoplastic lesions. Neoplastic lesions (principally pulmonary adenomas, lymphomas, hepatocyte lesions, liver vascular lesions) were found in all treatment and control sets. There was no indication that a specific anesthetic or anesthetic dose was carcinogenic. Our results do not confirm the suggestion that isoflurane is a hepatocarcinogen, nor do our data suggest that. the modern inhaled anesthetics pose a significant threat of carcinogenicity.