Aaron Silvester set the pace as the Thruxton action kicked-off, clocking a best lap of 1min 19.789secs to lead Declan Connell by 0.384secs.
Third quickest in the opening session was Asher Durham, followed by Harrison Crosby as Jamie Lyons rounded out the top five.
Pirelli National Junior Superstock Championship with Santander Consumer Finance, FP1 results:
1. Aaron Silvester (A&J Racing) 1min 19.789s
2. Declan Connell (DC Racing) +0.384s
3. Asher Durham (G&S Racing) +0.592s
4. Harrison Crosby (JR Performance Racing) +0.610s
5. Jamie Lyons (C&M Motorsport/Ability Energy +0.696s