The pharmacokinetics of ouabain associated with toxicity were studied in the cat and the guinea pig both in vivo and in vitro using ouabain-H3. After spinal cord transection a higher dose of ouabain was required to reach the lethal endpoint. This intervention also increased the myocardial and serum levels associated with death. These findings were corroborated in experiments using digitoxin H3. In vitro, substantially higher ouabain tissue contents were associated with a lethal event. In addition, in cats and guinea pigs, the lethal myocardial ouabain content did not change when the infusion rate of ouabain was varied in vivo or the perfusate ouabain concentration was changed in vitro. In vivo, propranolol increases the myocardial ouabain content associated with death to in vitro levels. In vitro, the drug prolongs the time to death by retarding the myocardial uptake of ouabain. These data suggest that the toxic effects of ouabain in the whole animal are largely neural and in the isolated heart, substantially myocardial.