The activity of a new i.v. anaesthetic agent ICI 35 868, a compound unrelated to currently used barbiturate, eugenol or steroid agents, has been examined in a range of animal species. Some of the properties of ICI 35 868 resemble those of thiopentone in that it is a rapidly acting agent which produces anaesthesia of short duration and without excitatory side-effects. Both agents have a similar therapeutic index and produce equivalent cardiovascular and respiratory effects. In the mouse ICI 35 868 is 1.8 times more potent than thiopentone as a hypnotic. However, the anaesthetic profile of ICI 35 868 differs from that of thiopentone in that recovery is rapid following repeated administration, no tissue damage is produced by perivascular or intra-arterial injection and greater reflex depression and more profound e.e.g. changes are produced at equipotent doses. This new agent has been shown to be compatible with a wide range of drugs used for preanaesthetic medication, inhalation anaesthetics, and neuromuscular blocking drugs.