Plasma immunoreactive gastrin (IRG) levels and abomasal acid secretion were investigated on cows with Heidenhain pouch. In two suckling calves (1 month), plasma IRG levels attained at a peak value of 120-160 pg/ml within 3-4 hours after suckling and decreased gradually to about 50 pg/ml by the next suckling. The titratable acidity rose to a peak value of 75-115 mM/l between 3 and 4 hours after suckling and then gradually decreased to the pre-suckling levels (about 20-40 mM/l) between 9-11 hours after suckling. These acid secretory patterns paralleled to the plasma IRG levels. The significant correlation was observed between plasma IRG and total acidity (p<0.01). In a weaned calf (4 months), plasma IRG and acid secretory responses after feeding were indistinct compared with those in suckling calves. Titratable acidity of pre-feeding was 70-110 mM/l and slightly elevated to about 130 mM/l, and subsequently reduced. The significant correlation was also observed between plasma IRG and total acidity (p<0.01). In an adult cow (20 months) fed with roughage only, post-prandial responses of plasam IRG was not observed and the pH value of the secretion is higher than 7.0.