This document reports on a study of the likelihood of individuals participating in the Federal food stamp program and the Medicaid program and the likelihood of exiting those programs. Data were analyzed from the first panel file of the Survey of Income and Program Participation (SIPP). Special problems with representativeness and measurement error are discussed. The following characteristics were found to be associated with participants: (1) males are less likely than females to participate and more likely to exit; (2) Blacks and Hispanics are more likely to participate than are Whites, but exit rates are similar for all racial groups; (3) those with less than a high school education are more likely than high school graduates or college graduates to participate; (4) those in a nonmarried-couple family were more likely to participate than those in married-couple families or persons not in a family; and (5) full-time workers had the lowest likelihood of participating. The following characteristics were found to be associated with those who exited: (1) those exiting programs seldom had changes in family status and marital status; (2) the majority of those who exited had no significant change in work activity; (3) exiting a program does not necessarily mean an improvement in economic status; and (4) almost one-half of those who exited the Medicaid program had failed to find a new source of health insurance coverage 12 months after leaving. Statistical data are included on six tables. Three references are appended. (FMW)