On the basis that the most repeatable measurement of acid output of the stomach is the peak half hour after an augmented dose of histamine, the author suggests a modification of the Kay test, including titration of the gastric juice to neutrality and measurement of the chloride concentration. The aspiration tube was sited fluoroscopically. It is suggested that the ;peak half hour' should be substituted for the period 15-45 minutes after administration of histamine. This modification is supported by the presented data. It should be noted, however, that the titration technique has also been modified. The ranges of basal and maximum gastric secretion (volume, acidity, acid output, parietal and non-parietal components) in 20 normal males and 20 normal females are presented and analysed for differences of age and sex. Both basal and maximum (histamine stimulated) gastric secretion declined with age, more so in women than in men. ;Maximum' gastric secretion was greater in men than in women, especially in older subjects.