The influence of an external drag to account for the secular decrease of the semi-major axis of LAGEOS is reexamined. We develop a theory for the neutral and electric drag more refined than those previously available. The charge drag appears to be the most likely cause of the secular decrease of LAGEOS of about 1.1 mm day-1. Likewise we show the existence of annual and semiannual variations connected with the orientation of the satellite's orbital plane with respect to the sun. Finally we express the electric drag as n5/6 T2/3 in terms of ion number density and absolute temperature. The rate of fall of LAGEOS constrains the range of possible physical states of the plasmasphere at altitudes as high as 6000 km.