1. Compliance with digoxin therapy has been assessed in a group of fifty patients receiving the drug in general practice but not attending hospital. 2. Compliance was estimated by comparing plasma digoxin concentrations before and after a 10 day period of measured digoxin consumption and by tablet counting. 3. Twelve patients had plasma concentrations which increased by more than 0.4 ng/ml during monitored intake and eight other patients took less than 80% of their tablets. These twenty patients were considered non-compliant. 4. A further three patients in whom plasma digoxin levels were zero when first seen but increased substantially during the run-in period were also adjudged non-compliant. 5. Non-compliance with prescribed digoxin dosage occurred, therefore, in 46% of the patients studied.