A preliminary hearing will be held April 1 for Dr. Donald Medaris, a 75-year-old physician who was arrested Feb. 22 at his office in Arcadia on 12 felony counts of violating state laws in connection with the prescription of controlled narcotics. The arrest followed a four-month undercover investigation by the Arcadia Police Department, the district attorney's office and the state Board of Medical Quality Assurance. Investigators say they were able to obtain prescriptions from Medaris for 1,000 tablets of controlled drugs, including tranquilizers and sleeping pills, without medical need. Apart from the criminal charges, Medaris will face an administrative hearing before the state medical board as a result of his arrest. The board placed him on probation from 1980 to 1983 for gross negligence and incompetence. The preliminary hearing will be in Santa Anita Municipal Court.