Comparison of phenytoin plasma concentrations in an intravenous and intramuscular crossover study in 12 healthy ambulant, male subjects indicates that phenytoin administered intramuscularly is absorbed over a period of approximately five days. A model simulating precipitation and redissolution of the drug at the injection site has been shown to satisfactorily fit observed plasma concentration data following intramuscular administration. It is proposed that this model will be useful in the selection of an appropriate dosing regimen in situations in which intramuscular administration of phenytoin is indicated.