Kieran Kent clocked the best time of the day as he posted a 1min 19.275sec lap to fend off a charge by Charlie Atkins.
James McManus was third quickest, ahead of Mikey Hardie as Kieran Smith was fifth.
HEL Performance British Junior Supersport Championship with Motocourse, Donington Park, Free Practice:
1. Kieran Kent, D&P Lovell Kent Racing
2. Charlie Atkins, Symcirrus Motorsport +0.352s
3. James McManus, Completely Motorbikes/Affinity +0.379s
4. Mikey Hardie, Completely Motorbikes/Affinity +0.630s
5. Kieran Smith, Smith Twins Racing +0.641s