The peak plasma level of Trihexyphenidyl sustained release from (THR = Parkinane sustained release) is lower than that produced by the simple form (THS = Artane) while it provides a quantitatively equivalent serum concentration. The less noticeable with sustained release Trihexyphenidyl. THR plasma levels, maintained relatively constant by a progressive release of the active principle, are sustained over a 24-hour period, allowing a simplified mode of administration: the drug may be given once a day.