We carried out the empirical investigation of the evolution of gas density distribution in the envelopes of planetary nebulae (PN). For this purpose we analysed the isophotal maps of 10 PN in the lines H alpha, H beta or in the optically thin radio continuum. To obtain the spatial radial distribution of gas density n(r) (where n = n(H)+n(He)) we used Abell's integral equation in the simplest, spherically-symmetric case. We found that n(r) for all PN envelopes in our sample can be described by an approximative expression: