Martinsville, VA (SportsNetwork.com) - Regan Smith substituted for Kyle Larson in the No. 42 Chip Ganassi Racing Chevrolet in Sunday's Sprint Cup Series race at Martinsville Speedway after Larson was sidelined due to additional medical tests following his fainting spell on Saturday.

Larson, the 2004 rookie of the year in NASCAR's top series, was taken to nearby Martinsville Memorial Hospital for precautionary reasons after he fainted during an autograph session at this half-mile track. He was awake and alert when he was transported to the hospital. On Saturday night, the 22-year- old Larson was sent to the Carolinas Medical Center in Charlotte, where he underwent further testing and consulted with a neurologist.

"Although all tests came back negative and Larson feels completely fine, the doctors felt he should be held for more testing today," Larson's team said in a statement.

Larson's team had put a plan in place if he would not be able to compete in this 500-lap race at Martinsville. The team contacted Smith on Saturday night and sent him to its race shop in Concord, North Carolina to be fitted for a seat in the No. 42 car.

"I pretty much just got wind that something was going on last night," Smith said during a press conference held shortly before the start of the race. "We had a plan in effect that if for some reason Kyle couldn't race today. I didn't know anything until this morning when I woke up. I had a text message when I woke up this morning and headed this way."

Due to the driver change, Smith had to start the race from the rear of the field.

Smith, who drives a full-time schedule for JR Motorsports in the Xfinity Series, has nine previous Sprint Cup starts at Martinsville but had not competed here since April 2013. He drove the No. 41 Stewart-Haas Racing Chevrolet in place of Kurt Busch in the first three races this season (Daytona, Atlanta and Las Vegas) when Busch was suspended by NASCAR amid domestic violence allegations. Smith has also substituted for Tony Stewart and Dale Earnhardt Jr. in the past.

Even though Larson missed this race, he remains eligible for the Chase for the Sprint Cup championship since he had qualified seventh for the event on Friday. Larson came to Martinsville ranked 18th in the point standings. He has scored two top-10 finishes in five races this season.