Luke Mossey led the way as first practice got underway at Thruxton, taking charge with a hot lap of 1min 15.954secs.
Mossey led Tom Oliver by 0.613secs at the end of the session, as Scott Swann was third.
Shaun Winfield was fourth for the TAG Racing Honda squad as David Allingham rounded out the top five.
Pirelli National Superstock with Moneybarn Vehicle Finance, FP1:
1. Luke Mossey (Milenco by Padgetts Motorcycles) 1min 15.954s
2. Tom Oliver (Armada Marine Cables Racing) +0.613s
3. Scott Swann (Swann Racing) +0.631s
4. Shaun Winfield (TAG Racing Honda) +0.869s
5. David Allingham (SMS/Nicholls Oil Racing) +0.992s