The first results of a study of F-type MS stars using the He I D3 absorption feature at 5876 A as an indicator of stellar activity and structure are presented. The observations were obtained with the QUB echelle spectrograph at the Cassegrain focus of the 4.2 m William Herschel telescope. Seventeen F-stars of the Hyades main sequency were chosen as a first sample. It was found that He I D3 and X-ray emission appear around B - V = 0.3, indicating the onset of sub-surface convection zones. Below this value, in stars with B - V not greater than 0.42-0.44 the behavior of the He I D3 equivalent width against Rossby number is different from that observed in later type stars, indicating that the presence of nonradiative mechanisms other than dynamo must be important in heating the chromospheres of these hotter stars.