The purpose of this study is to explain the changing and increasing participation of females in the American labor force during this century, especially since 1940. Although the pre-1940 changes are smaller and may be attributable to improvements in census enumeration techniques, there has been a significant increase since 1940 in participation rates of married women in their 30's or older. Noting that the demand for female labor has increased over the years, mainly because of growth in typically female occupations, the study concludes that the increased female labor force participation is due to increased demand, which in turn has induced increased supply, especially among women whose children are in school. (BH)