In this note I revisit Heckman's proposal, [Heckman, 1974], to specify a static labour supply model using a simple formulation for the Marginal Rate of Substitution between total expenditure on consumption and hours of work. I describe the analytical form of the expenditure function and I show how the direct and indirect utility functions can be recovered. I propose an alternative specification for the MRS and in this case I describe analytically the labour supply functions, the indirect and direct utility functions as well as the expenditure function.