Abstract Emphasis is placed on the simplest models of the chemical evolution of galaxies, and the role played by the concentration of the interstellar gas to gravitational stable regions is taken into account. Although Schmidt's simple model is improved when concentration is allowed for, it still predicts too wide a spread of metal abundances for agreement with the observations. An exactly solvable accretion model in which the gas mass rises to a maximum and then declines gives the smaller spread observed. This model gives a rather prompt initial enrichment automatically. Finally the models with a linear relationship between star formation rate and gas mass are used to demonstrate effects occurring when the cooking times of elements are taken into account.