Nonanesthetized cats of both sexes were given oral digoxin (0.011 mg/kg of body weight) 3 forms: elixir, tablet, and crushed tablet mixed with food. Mean peak plasma concentrations of digoxin were highest with the elixir (1.89 +/- 1.02 ng/ml) and lowest with the crushed tablet mixed with food (0.66 +/- 0.35 ng/ml). Male cats had significantly higher (P less than 0.10) mean plasma digoxin concentrations than did female cats. A 2nd group of nonanesthetized cats of both sexes was given digoxin elixir orally at therapeutic amounts (0.011 mg/kg) once a day for 4 consecutive days. The cumulative effect of digoxin resulted in 62% increase in the mean peak plasma concentration and 231% increase in the 24-hour plasma concentration of digoxin over the 4-day period. Male cats had a significantly (P less than 0.05) higher mean plasma digoxin concentration than did the female cats. Significant changes in the ECG were not recorded. A 3rd group of nonanesthetized cats of both sexes was given a single toxic dose (0.11 mg/kg) of digoxin elixir orally. All cats showed clinical signs of digitalis toxicosis (depression, vomiting, salivation, and anorexia) before ECG changes appeared. Alterations in the ECG were minimal; the most important changes were a slight increase in the PQ interval, an elevated ST segment, and decreased heart rate. Plasma concentrations of digoxin at the time of vomition ranged from 4.45 to 12.12 ng/ml with a mean peak plasma value of 7.37 +/- 3.61 ng/ml. The cats were clinically ill for 48 to 96 hours. A plasma digoxin concentration of 2.3 ng/ml was not toxic.