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Urethane is often used as a sole anaesthetic agent for non-recovery studies in laboratory animals. However, the use of urethane is controversial, in part, because the electroencephalogram after urethane administration is similar to the electroencephalogram recorded from unanaesthetized animals. Here, we assessed the minimum alveolar concentration (MAC)-sparing effects of urethane by measuring the effect of two doses of urethane on the MAC of isoflurane in male Sprague Dawley rats. Isoflurane MAC was measured before and after intravenous administration of urethane at 1.0 g/kg (Group G₁, n = 6) and 1.5 g/kg (Group G₁.₅, n = 6), or an equal volume of 0.9% saline (Group Gs, n = 6). Baseline isoflurane MAC was not statistically different between groups (isoflurane concentration: 1.47 ± 0.08%, 1.40 ± 0.19% and 1.42 ± 0.12% for G₁, G₁.₅ and Gs, respectively). Intravenous injection of saline did not alter isoflurane MAC (post-saline MAC: 1.43 ± 0.11%). After urethane administration, isoflurane MAC decreased in a dose-dependent manner (new MAC G₁: 0.19 ± 0.06%; G₁.₅: 0.03 ± 0.01%; P < 0.05). The isoflurane MAC after 1.5 g/kg urethane was not significantly different from room air isoflurane concentrations (0.01 ± 0.01%), demonstrating a 100% MAC reduction at this dose. In conclusion, high-dose urethane (1.5 g/kg intravenously) was suitable as a sole anaesthetic agent to prevent gross purposeful movement during the conditions of the study, whereas low-dose urethane (1.0 g/kg intravenously) was not.

Urethane (ethyl carbamate) is used alone or in combination with other drugs to produce anaesthesia in laboratory animals. Although originally studied as a potential phytocide, urethane demonstrated antineoplastic properties when administered to rats with the Walker rat carcinoma 256. Subsequent trials in humans led to its use as a chemotherapeutic agent for various leukaemias. Mice develop pulmonary adenomas earlier in life and at a higher incidence following urethane administration. Urethane's carcinogenic influence is greater in neonatal mice; it also has a transplacental influence in mice. In rats, urethane increases the incidence of pulmonary adenomas, Zymbal Gland tumours, and a variety of other neoplasms. Urethane is absorbed sufficiently from the skin of laboratory animals to produce a transient narcosis. The carcinogenic effect appears to be due to an undefined oncogenic intermediate formed in the blood. Considering the properties urethane demonstrates in animals, the safety of its use by laboratory personnel is in question. However, if appropriate guidelines are followed, urethane should continue to be a useful anaesthetic agent for laboratory animals.