To compare two protocols of combined parenteral general anesthesia, the authors analyzed electrocardiographic changes in anesthetized rats undergoing left pneumonectomy. One group of rats was anesthetized with a combination of medetomidine and ketamine (group 1, n = 10), and the other was injected with diazepam and ketamine (group 2, n = 10). Investigators obtained two electrocardiograms from each rat, one before surgery (5 min after anesthesia) and one after surgery (60 min after anesthesia). Anesthetic induction was quick for all rats, though four rats in group 2 died before surgery. Mean cardiac frequency and R-wave amplitude were significantly lower in rats in group 1 than in rats in group 2. Rats in group 1 received injections of atipamezole about 60 min after surgery, which reversed the effects of medetomidine; these rats regained voluntary respiratory movement more quickly than did rats in group 2. Two additional rats in group 2 died during postsurgical recovery. These results suggest that for thoracic surgery in rats, medetomidine-ketamine is an appropriate anesthetic combination, may be safer than diazepam-ketamine and yields a shorter recovery time.