Oportunidades, a conditional cash-transfer program instituted in Mexico in 1997, provides cash incentives to mothers to invest in the health and education of family members. Drawing from data gathered by Mexico's National Institute of Public Health, this study assesses the effect of the program on contraceptive use and birth spacing among titulares (female household heads) living in rural areas during the experimental period, 1998-2000, and during 2000-03, after incorporation of the control group. In 2000, titulares were more likely to use modern contraceptives than were women in the control group, although by 2003 all beneficiaries had the same probability of use. Change in autonomy was not a mediator, although baseline autonomy modified the program's influence on contraceptive use. Cox proportional hazard models produced estimates that birth spacing was similar between the beneficiaries and controls. Inconsistent findings may be the result of the way contraceptive use was defined in this study. Findings from this study may be useful for helping program planners better understand the role of conditional cash transfers in modifying family planning and fertility among poor rural women in Latin America.