Milk volume, day of initiation of pumping, duration and frequency of milk expression, and length of night rest interval were evaluated for the first month postpartum in 32 healthy, nonsmoking women (ages 20 to 38 years) who delivered at 28 to 30 weeks' gestation. Early milk volumes were related negatively to the delay between delivery and the initiation of milk expression (r = -.48, P less than .02). Average milk volumes at 2 weeks and 4 weeks postpartum were 493 +/- 330 and 606 +/- 369 mL/d (mean +/- SD), respectively, and were not related to the absolute frequency or duration of pumping or to night rest interval. Volume changes between weeks 2 and 4 postpartum were correlated with the absolute frequency (r = .49, P less than .01) and duration of pump use (r = .42, P less than .05) during this interval and with changes in frequency (r = 0.56, P less than .002) or duration (r = 0.49, P less than .05) between the first 2 and the second 2 weeks postpartum. Optimal milk production was associated with five or more milk expressions per day and pumping durations that exceeded 100 min/d.