The electron density profile in the F1 region is studied based on data from rocket experiments, and profiles for summer and winter during low and medium solar activity and for the intermediate season during low activity are presented. The f0F1 data are used to confirm the reliability of the profiles.A theoretical study is made on the formation of the F1 region and calculated profiles for all the above cases are given. The agreement with the experimental curves is generally good.The meaning of the so-called maximum electron density (Nm) computed from f0F1 is discussed, and some argument is given on the unreasonableness of the customary expression of the Nm as a simple function of R (Wolf number) and χ (solar zenith angle).Finally the condition that a maximum in N profile can take place is discussed.