Ten puerperal women were put on a 1,200 kcal diet per day for 3 days followed by an ordinary 1,800 kcal diet per day for 3 days [Low-Caloric (LC) group]. Fifteen puerperal women were put on an ordinary 1,800 kcal diet per day for 6 days[Ordinary-Caloric (OC) group]. All of the women took breakfast at 7:30 am, blood was drawn at 9:30 am and then the breasts were milked for 2 minutes with an electric breast pump starting on the day of delivery (Day 0) and continuing to Day 5. Four women in the LC and OC groups underwent a thyrotropin-releasing hormone (TRH) stimulation test on Days 1 and 3. Serum prolactin (PRL) and TSH levels in the LC group tended to be much lower than those of the OC group during the test period. Serum PRL and TSH release response to TRH in the OC group were much higher than in the LC group on Days 1 and 3. No significant difference was noted between the OC and LC groups in serum estradiol (E2) and cortisol levels from Day 0 to Day 5. The milk volume in the LC group was significantly less than in the OC group on Day 1. Analysis of milk specimen components revealed little difference between the OC and LC groups. These results suggested that a low-caloric diet may suppress PRL, TSH and milk secretion and may not be useful for breast-feeding.