Arterial blood pH and blood gas tensions were measured before and after feeding in seven young (1-year-old) and 6 aged (10 to 12 year-old) conscious beagle dogs with a portable "i-STAT" blood gas analyzer to obtain the fundamental physiological data on the laboratory dogs. Before feeding, arterial blood pH, HCO3- and base excess (BE) were 7.395 +/- 0.022, 20 +/- 1.0 mmol/L and -4 +/- 1.1 mmol/L in young dogs, and 7.408 +/- 0.031, 23 +/- 1.5 mmol/L and -2 +/- 1.8 mmol/L in aged ones. After feeding, the above values changed to 7.449 +/- 0.021, 25 +/- 1.1 and 1 +/- 1.3 in young dogs, and 7.456 +/- 0.022, 28 +/- 1.7 and 4 +/- 1.9 in aged ones. After feeding, each of the values for pH, HCO3- and BE was increased in comparison with those before feeding in both young and aged dogs, and alkalinization of blood, the alkaline tide, was observed in all healthy dogs. The values for PCO2 also rose from 33.6 +/- 1.91 to 36.7 +/- 1.95 (mmHg) in young dogs, and 36.4 +/- 2.97 to 40.3 +/- 3.06 (mmHg) in aged ones, and in 3 of 6 aged dogs slight hypoxia (PO2 < 80 mmHg) was noted. These changes in PCO2 and PO2 were considered to be due to compensatory hypoventilation for the alkalinization of blood. To confirm the cause of the alkaline tide phenomenon, we investigated the association of the alkaline tide with gastric acid secretion by cimetidine, an H2-blocker; and found that it was effective to inhibit postprandial alkaline tide in dogs. After feeding, ionized Ca concentration (iCa) was decreased inversely with an arterial blood pH increase in both young and aged dogs. The mean values of iCa in aged dogs after feeding was lower than those in young dogs. In the present experiments, it was demonstrated that in beagle dogs the acid-base balance and blood gas values were largely affected by feeding, especially in aged dogs. In case blood gases would be evaluated in toxicity studies, we should consider the influence of feeding and aging.