Prerequisites for proper measurement of gastric mucosal blood flow (MBF) are different according to whether the blood supply of the stomach is being recorded in experimental animals or in humans. In this review the pros and cons of the methods to measure MBF are analysed systematically. Although it is easier to find a procedure for experimental animals none of the method hitherto published proved to be quantitatively accurate for the determination of MBF changes and simultaneous variations in gastric acid production. It is even more difficult to elaborate a method for human studies which should not cause serious inconvencience for the patient. The most one can achieve is a sufficiently accurate estimate of the MBF and its changes in the course of the study. With the help of such procedures it is possible to clarify the interrelation between MBF and acid secretion using interventions or bioactive substances which influence either the mucosal circulation or parietal cell function or both.