In our country, patients with congestive heart failure who are treated chronically with digoxin are usually advised by their physicians to stop taking the medication two days a week. This is probably aimed at decreasing digitalis toxicity. Based on digoxin pharmacokinetics we assumed that the drug plasmatic level should diminish by 40 to 50%, below the therapeutic concentration of 0.8 to 2 milligrams, after two days of suspension. The objectives of this study were: a) to analyze the reduction of the plasmatic concentration of digoxin after a two day interruption of treatment, b) to compare the plasmatic levels of the drug between patients who received continuous and discontinuous treatment. A prospective, randomized and simple blind trial was designed. A total of 36 patients with congestive heart failure and systolic dysfunction with atrial fibrillation or sinus rythm were included. Group 1 (19 patients) received continuous treatment and Group 2 (17 patients) took the drug from Monday to Friday. In the continuous treatment group there was no significant difference between the Monday (1.06 +/- 0.55 milligrams) and the Friday (1.1 +/- 0.57 milligrams) digoxin concentrations. In the discontinuous treatment group the Monday digoxin concentration (0.611 +/- 0.396 milligrams) was lower than the Friday one (1.04 +/- 0.58 milligrams). The difference was statistically significant with a p = 0.000002. In conclusion, the two days a week suspension schedule reduces the plasmatic concentration of digoxin to subtherapeutic levels while the continuous regime maintains stable concentrations within the therapeutic range. Adjusting the dose to the creatinine clearance, average concentrations of 1 milligram are obtained. These results suggest that digitalis intoxication could be prevented by adjusting the dose according to renal function rather than interrupting the treatment as it is usually done in our country.