The relations between fertility, mortality, growth rate and age distribution in closed populations have been derived by means of a set of differential equations based on the well known theory of chemical kinetics. The classical relations for stable populations are easily obtained in this model by simple algebraic manipulations. A rough but useful further simplification is to divide the population into three groups—pre-reproductive, reproductive, and post-reproductive. For this three-group model simple algebraic expressions connect fertility, mortality, growth rate and the fractions of the population in each group. Although the relations obtained are not precise, they serve to illustrate simply and directly the interactions among the basic population variables.