Peter J. McLaughlin, 45, the register of wills in the D.C. court system since 1966, died of leukemia Thursday at Sibley Memorial Hospital.

As register of wills and clerk of the probate division of the D.C. Superior Court, Mr. McLaughlin was responsible for administering and auditing the estates of residents of the city who had died. In addition to supervising more than 60 employes, he gave legal and administrative advice to lawyers and others in an important area of the law that rarely receives wide public notice.

Over the years, Mr. McLaughlin testified before numerous congressional committees on legislation concerning probate and before the D.C. City Council, which passed a Probate Reform Act in 1980.

In a statement issued after Mr. McLaughlin's death, Chief Judge H. Carl Moultrie I of the Superior Court praised him for spending "hundreds of hours of his own time during the past year to ensure a smooth transition from the old statute to the new probate law. The bench, the bar and the citizens of the District of Columbia are in his debt for these efforts."

When Mr. McLaughlin was named register of wills, estates were probated in the U.S. District Court for the District of Columbia. In 1973, this function was transferred to the D.C. Superior Court. Mr. McLaughlin began his career as an officer of the court as deputy register of wills in 1963. With the retirement in 1966 of Theodore Cogswell as register of wills, Mr. McLaughlin was named to succeed him by Matthew F. McGuire, then the chief judge of the U.S. District Court. He retained that post at Superior Court.

Mr. McLaughlin was born in Syracuse, N.Y., and graduated from Le Moyne College there in 1957. In the same year, he moved to Washington to enroll at Georgetown University law school, and earned his degree in 1961. He was a trust officer with the National Savings and Trust Company before joining the courts.

Mr. McLaughlin, who lived in Washington, was a member of the D.C. Bar, the Bar Association of the District of Columbia and the Barristers. He was a board member of the D.C. Lung Association, a treasurer of the Animal Rescue League of the District of Columbia, and a member of the City Tavern Club.

Survivors include his mother, Mrs. Daniel J. McLaughlin, two brothers, Harold and Paul, and two sisters, Betty McLaughlin and Margaretta Cooney, all of Syracuse.

The family suggests that expressions of sympathy be in the form of contributions to the Animal Rescue League of the District of Columbia.