We have measured the He I D3 line 5876 Å, in 32 F-type young open cluster stars and in 4 field stars. Combining our results with previous observations of D3 we strengthen the following conclusions: (a) Sub-surface convective zones appear, on the main sequence, around F0 (B - V 0.3). (b) The stars between F0 nd F5 (0.3 B - V 0.42) are chromospherically active irrespective of age, which implies a heating mechanism not affected by evolutionary effects. In this range the activity in D3 and X-rays are uncorrelated. (c) For stars later than F5 the correlation among all the activity indices, chromospheric and coronal, shows a common energy source. Convective zones here should be deep enough to generate magnetic fields capable of supplying this energy, and also for braking the rotation.