Lactation was prolonged in 16 rats up to 70 days after parturition by providing them with fresh litters every 10 days. Milk production, as judged by litter weight gains, declined about 1/3 after the first 20 days and continued at this level for the remainder of the 70-day period. The lobule-alveolar system of most rats showed little or no evidence of involution during entire period. This suggests that the decline in milk yield was due either to decrease in secretion of galactopoietic hormones or to reduction in secretory capacity of alveolar cells, or both.