In this paper we propose a model for the chemical evolution of the solar neighbourhood and the galactic disk as well. The model takes into account the inflow of primordial gas, and the time scale associated with the process of disk formation. We follow the same physical description and formalism adopted by Chiosi (1979). However, by releasing the approximation of instantaneous recycling, we are able to describe the chemical history of the galactic disk in much better detail than in Chiosi (1979). A set of eight elements (H,D,He3,He4,c+0,N,n-rich, heavy) is used to describe the chemical composition of the galactic gas and inflowing material. The disk is supposed to be represented by several independent shells, whose surface mass density is thought to depend on radial distance from the galactic center and age of the galaxy. The rates of mass accretion and star formation follow the formulation of Chiosi (1979). To better approximate the results from dynamical models of disk galaxies, we have taken into account the known fact the time scale of disk formation varies with the distance from the galactic center, being of the order of 1×109 y in the innermost, and up to 10×109 y in the outermost, regions of the disk. From the body of solutions we have explored, we can derive the following results, which are shortly summarized for the case of our best model of chemical evolution of the disk only.