The Longitudinal Study of Social, Economic, and Demographic Change was undertaken during the late 1960s to provide detailed national information on behavior and attitudes related to fertility and family planning in Thailand. Results from the second round of the study indicate that the practice of family planning increased substantially in both rural and urban areas during the three-year interval between the two rounds. During this period, marital fertility registered a decline in the urban areas. This was a result of a small rise in fertility among Bangkok-Thonburi women combined with a sharp decline in fertility among provincial urban women. Because the National Family Planning Program was officially begun at the time of the first round, its role in the increase in contraceptive use is examined. Although the study shows that the desired number of children is substantially below the actual number of children Thai womenhave by the end of their reproductive years, the desired number of children is still well above the number usually considered ideal in developed countries. Nonetheless, changes in reproductive behavior appear to reflect the impact of modernization on Thai society. In urban areas, marital fertility is only moderately high and a substantial proportion of couples practice family planning. Of particular importance over the remainder of the decade will be the reproductive behavior of rural women, who constitute an extremely high proportion of the Thai female population.