A Northern Virginia businessman shot a would-be burglar and then was shot himself early yesterday after two assailants surprised him on his 25-acre Loudoun County home, police said.

Loudoun sheriff's investigator John Sheldon said John H. Crowe Jr., 31, head of Crowe Enterprises, an international marketing firm based in Alexandria, was jumped shortly after midnight by two men hiding in shrubbery near a side entrance to his brick farmhouse north of Leesburg.

After a brief struggle, Sheldon said Crowe managed to reach a handgun he kept in a bedroom drawer and wounded one of his attackers. A suspect then seized the weapon and shot Crowe in the head. The assailants fled with the gun, Sheldon said, leaving "a trail of blood down the driveway."

Crowe was airlifted by Maryland State Police helicopter from the farm in the northeast corner of Loudoun to the shock-trauma unit of Suburban Hospital in Bethesda. He was listed in critical condition yesterday after three hours of surgery for his head would.

Sheldon said police interviewed Crowe both before and after surgery and that Crowe furnished descriptions of both attackers and a getaway vehicle. Sheldon said Crowe, whose wife and two children were visiting relatives in Nags Head, N.C., at the time, did not know the suspects.

According to Sheldon, investigators believe the motive for the attack was robbery. A large quantity of silver on a table in the brick farmhouse appeared untouched, he said, but at least one item was believe missing. Sheldon said an alarm system in the house was deactivated by Crowe when the pair forced him inside.

Investigators recovered one slug from the ceiling of Crowe's residence and found another in a quilt on the bed. Sheldon said police speculated a third slug might have struck the wounded suspect.

A family friend, Jack Spencer of Alexandria, said Crowe had been out with friends earlier in the evening before returning to his isolated, country home on Rte. 661 east of the crossroads community of Lucketts. Crowe moved to Loudoun from Fairfax County about a year ago, he said.

Spencer said that besides Crowe Enterprises, Crowe also is associated with Amway Corp., which sells soap and other domestic products through a nationwide network of independent distributors.

Amway spokesman John Gartland said yesterday Crowe had achieved the rank of "diamond" distributor, meaning he is one of about 400 Amway dealers who have at least six other distributors under their supervision.

Burglaries have increased dramatically this year in Loudoun, particularly in the county's more densely populated eastern end. Lt. Leonard McDonald said yesterday the sheriff's department had recorded 214 burglaries by the end of May, compared with 174 for all of 1980.