The front two were locked in a battle to record the fastest time throughout the 45-minute session at Snetteron, with Buchan eventually ending up on top.
It was a good day for Massingberd-Mundy Kawasaki, with Lee Jackson finishing third, a shade over a second behind Ryde.
The second free practice session takes place on Saturday, followed by qualifying, with three races spread across the weekend.
This is the second event of the Covid-19-affected season, with Honda Racing duo Glenn and Andrew Irwin leading the Bennetts British Superbikes standings after the first round, which took place at Donnington Park two weeks ago.
1. Danny Buchan (Massingberd-Mundy Kawasaki) 1m:49.119s
2. Kyle Ryde (Buildbase Suzuki) +1.003s
3. Lee Jackson (Massingberd-Mundy Kawasaki) +1.011s
4. Andrew Irwin (Honda Racing) +1.123s
5. Christian Iddon (VisionTrack Ducati) +1.209s
6. Glenn Irwin (Honda Racing) +1.229s
7. Ryan Vickers (RAF Regular and Reserve Kawasaki) +1.295s
8. Josh Brookes (VisionTrack Ducati) +1.441s
9. Jason O'Halloran (McAMS Yamaha) +1.948s
10. Hector Barbera (Rich Energy OMG Racing BMW) +2.162s