Digoxin, frequently used in the treatment of congestive heart failure, has a very narrow therapeutic index. We studied the differences in digoxin pharmacokinetics when ingested in the morning versus evening. A single digoxin (0.25 mg) dose was given orally to the same group of 10 diurnally active healthy (6 male and 4 female) volunteers in the morning at 08:00 and evening at 20:00 in separate experiments scheduled 2 weeks apart. Blood samples were collected at specific times for 48h after each timed dose; digoxin was determined by radioimmunoassay (RIA). Maximum plasma concentration C-max; T-max, the time to reach C-max; area under plasma concentration curve AUC; and elimination half-time T-1/2 of digoxin were determined. T-max was statistically significantly shorter (54 min) following 08:00 dosing compared to 20:00 dosing (96 min). Although the Cm was higher after morning than evening dosing, it was not significantly so. No other parameter of digoxin pharmacokinetics except Tm exhibited administration time dependency.