Physiological diarrhea of baby during the period of breastfeeding often occurs. To investigate the relationship between mother's milk and the physiological diarrhea of baby, we measured the PGE2 levels in milk of 320 lactating women within 4 months of postpartum with radioimmunoassay and the mean value of PGE2 level was 216.8 +/- 145.2 ng/L. The PGE2 levels in mother's milk in the group with diarrhea were 286.5 +/- 142.2 ng/L, that of control group 130.4 +/- 76.3 ng/L. The difference was obviously significant (P < 0.001). The physiological diarrhea of baby was positively related to the PGE2 level in mother's milk (r = 0.75, P < 0.01) i.e., the high PGE2 level in mother's milk may be an important cause of diseases. The observation on 102 cases of baby with severe physiological diarrhea showed a higher level of PGE2 in the mother's milk. 52 cases of lactating women in the treated group were given indomethacin, and after treatment the PGE2 level in mother's milk decreased obviously, and the diarrhea of baby disappeared. The effective rate was 96.15%. No side effects were found in both mother and baby.