Twenty patients received dipyridamole by two different dosage regimens yielding a total daily dose of 150 mg, either as 50 mg three times a day or 75 mg twice a day. The rationale for comparing these two regimens is that dipyridamole is usually given three times a day, but recent studies have revealed a final elimination half-life of the drug of about half a day. Based on drug cumulation during chronic dosing, the final half-life of dipyridamole observed in this study also averaged about half a day. The 75 mg b.i.d. regimen did not result in lower trough concentrations than the 50 mg t.i.d. regimen. There was wide interpatient variability in observed plasma dipyridamole concentrations for both regimens, averaging about 10-fold. These results suggest that dipyridamole could be administered twice a day and that dipyridamole levels should be monitored in clinical studies on the antithrombotic effect of the drug.