PIP: The life table method is used to analyze the process of family formation in Puerto Rico. The method approaches family formation in a cohort of women as a series of transitions between successive statuses in which each transition has a measurable probability of occurring. The method allows detained analysis of changes in fertility by separating changes in the intensity or number of births and the timing or spacing of births. The data were from the 1982 Puerto Rico Fertility and Family Planning Assessment Survey, a retrospective survey with detailed information on 3175 women aged 15-49. The analysis is limited to women ever in union. The births occurred during a period of more than 30 years, from around 1950-82, during which the total fertility rate declined by almost 50%, from 5.2 to 2.7 children/woman. Each birth interval was divided into 4 age groups to avoid the overrepresentation of women marrying younger and with shorter birth intervals. Analysis of the 1st 5 birth intervals for the 4 age categories in each clearly shows that the fertility reduction was reflected in the reduction in higher order births, a reduction already clearly visible in the transition to the 2nd birth. But contrary to the pattern in other low fertility populations, in Puerto Rico there does not seem to be a very consistent relationship between the decline in the probabilities of transition between birth orders and a more prolonged spacing. Women who reach each interval at the younger ages have a higher probability of having another child, and they do it more rapidly, which indicates the close relationship between age at the beginning of family formation, the size of the family, and the speed of the reproductive process. But even among older women the intervals are still very brief, indicating a pattern of rapid spacing independent of the number of children already born and perhaps of desired family size. The pattern indicates a clear distinction between behavior associated with terminating childbearing and that of spacing births. An analysis of changes in the intensity and calendar of births of different orders in 3 time periods, 1965-1969, 11970-1974, and 1975-1982, shows a clear trend toward longer intervals after 1965. Women who began childbearing at younger ages had shorter intervals than those who began later. In view of the abundant existing evidence on the adverse effects of short birth intervals, the process of family formation in Puerto Rico should be studied in greater detail, especially considering its possible impact on infant mortality.