PRIMIDONE (Mysoline) has been shown to be an effective anticonvulsant drug.1-6 It has been stated that primidone is exceptionally valuable in the control of focal seizures and the "drug of choice" for the treatment of psychomotor seizures.7-9 Previous clinical comparative studies have not contradicted this claim.4,5 One study found that phenobarbital and primidone were equally effective as anticonvulsants with 50 mg of phenobarbital being equivalent to 250 mg of primidone. Another similar study5 revealed that primidone and phenobarbital are about equally effective as anticonvulsants but more effective than diphenylhydantoin (Dilantin). However, as the authors of that study point out, the five day medication trial was not sufficient for maximal anticonvulsant effect of diphenylhydantoin to occur. Their data revealed a sharp increase in effectiveness of diphenylhydantoin on the fifth treatment day, while primidone and phenobarbital were relatively more effective on the first and second days. Not