Full-day center-based child care has repeatedly been associated with rising levels of cortisol, a hormone that helps the body manage challenge, across the day at child care. This article presents findings from two studies examining the relationship between child care program structure (number of days per week, and hours per day) and cortisol production across the day. Study 1 presents findings comparing cortisol production in 3- to 5-year-old children enrolled in either full-day (N = 55) or half-day (N = 63) Head-Start-funded programs. Study 2 presents findings comparing young children enrolled in either full-day full-time (5 days per week; N = 37) or full-day part-time (2-3 days/week; N = 41) primarily tuition-funded programs. Using multilevel modeling and controlling for a number of child factors, attending full-day, full-time programs (as compared to either half-day or part-time programs) was associated with increased cortisol production across the day on child care and home days. Implications for early childhood educators are discussed. (author abstract)