SummaryThe use of carbon dioxide (C0 2 ) with, and without, oxygen (0 2 ) as a short-term restraint anaesthetic for Wistar rats in which subclinical respiratory disease was endemic, was assessed in 3 separate experiments. In the first, rats were placed in a CO 2 atmosphere generated from solid CO 2 chips in a 70 Iplastic bin, and removed at time intervals ranging from 0 to 120 s after disappearance of the pedal reflex. Eight of 25 rats died, including 2 which were removed immediately the pedal reflex disappeared; it was concluded that CO 2 without O 2 was not a suitable short-term anaesthetic for rats. In a second study, rats were anaesthetized in atmospheres of 50 : 50 and 80 : 20 (C0 2 : O 2 ) provided from commercially available cylinders, in 2 different environments -a 3, 41 glass jar and a 171 plastic bin. Rats became excited in the plastic bin but not the glass jar. Rats in the glass jar displayed visible depression and cessation of whiskers movement significantly more quickly in the 80: 20 (C0 2 : O 2 ) than in the SO: 50 mixture (4'2±0'98s, n=6, and 66·0±4·9s, n=6 vs 13·8±2·77s, n=5 and 152·0±20·8s, n=5, respectively). Rats in the 171 plastic bin lost their pedal reflexes in a mean 41· 5 ± 4· 55 s (n = 11) in the SO : SO mixture and in a mean 30·9 ± 6·38 s (n = 11) in the 80: 20 (C02: O 2 ) group. Those left in the 50 : 50 mixture for 60 sand 180 s after disappearance of their pedal reflexes, recovered these reflexes in 20· 2 ± 0,44 sand 21·5 ± 7· 23 s respectively after removal from the gas. Respiration and heart beat ceased in one rat remaining in the SO : 50 mixture after 13 min 10 s. Received 12 March 1987. Accepted 25 November 1988 No untoward effects occurred in rats left in the 50 : 50 mixture for 180 s after disappearance of the pedal reflex, but 2 died when left for an equivalent period in the 80: 20 mixture. In the third study, examples of the practical use of a 50 : 50 mixture as a short term restraint anaesthetic are described. It was concluded that this mixture was a cheap, safe, and effective means of short-term restraint for rats with subclinical respiratory disease, when the minimal time of exposure to the gases was employed.