Observations on munition workers, most of them women, are organised to examine the relationship between their output and their working hours. The relationship is nonlinear: below an hours threshold, output is proportional to hours; above a threshold, output rises at a decreasing rate as hours increase. Implications of this finding for the estimation of labor supply functions are considered. The findings also link up with current research on the effects of long working hours on accidents and injuries.This article is protected by copyright. All rights reserved.