Top jumping riders from throughout the United States are expected for this year's Michelob Upperville Jumper Classic, to be held on the final day of the Upperville Colt and Horse Show at 2 p.m. Sunday.

This event is part of the American Grand Prix Association tour.

The $10,000 Pimm's Cup Hunter Classic also will be part of this year's competition.

The classic will take place on the historic Grafton Farm show grounds on Rte. 50 near Upperville, which is one hour's drive west of Washington. Tickets during the week are $3 per adult and $1 per child. Special rates are available for groups and box seats. For more information, call (703) 687-5634.

Celebrate the 200th anniversary of the U.S. Constitution by exercising your right to vote. Fairfax County residents can register from 8 a.m. to 4:30 p.m. Monday through Friday at the Office of the General Registrar, 4031 University Dr.

County groups of 10 or more may request the attendance of a registrar at their gatherings by calling 385-8100.

Home registration is available by request for citizens confined because of physical disability or lengthy illness.

Call 385-VOTE for a recorded message on all registration locations in Fairfax County, or 385-8100 for voter registration information and assistance.

The Mount Vernon Chamber Orchestra will honor its late conductor H. Stevens Brewster Jr. with the performance of a new work, "Serenade,"at 8 p.m. Sunday in St. Aidan's Church.

The work, composed by Stephanie Patterson to honor her former teacher, uses five of Brewster's poems as its text and will be sung by Metropolitan Opera baritone Charles Williams, a native of Alexandria. Brewster's youngest son, Lee, a violinist and senior at Mount Vernon High School, will perform the solo violin part, and Brewster's former colleague and friend Edward Skidmore of the National Symphony will perform the solo bass line.

Patterson says "Serenade" is "both a journey through and a celebration of life."

St. Aidan's Church is at 8531 Riverside Rd., Alexandria. Donations will be accepted at the door. For directions or more information, call 360-4953.

Volunteers are being sought for Reston Hospital's first summer teen program for students ages 14 to 18.

The program, which runs from June 22 through Aug. 28, asks qualified teen-agers to serve throughout the hospital during the week and on weekends in four- to six-hour shifts. They will work at the information desk, in physical therapy, the outpatient, pharmacy and medical/surgical departments, and in the gift shop.

For further information, call Volunteer Services Director Jane Kornblut at 689-9000.