In this paper the impact of working hours on the gross hourly wage rate of West German women is analyzed. We use a simultaneous wage-hours model which takes into account the participation decision. First, our estimates show that the hourly wage rate is strongly a¤ected by the working hours. In order to avoid any assumptions about the functional form, we estimate linear spline functions. Second, we detect di¤erent wage-hours profiles for specific groups of individuals. Despite these di¤erences, the wage reduction for jobs with less than 20 hours a week and for overtime hours turns out to be a robust result. However, the hourly wage rate of jobs with 20 to 38 hours does not di¤er significantly. Third, for West German women, the exogeneity assumption of working hours in the wage regression must be rejected if the wage-hours locus is assumed to be the same for all individuals. As a result of this the wage rate of full-time employees is overestimated in the standrad OLS estimation.