It has been previously documented that aspirin induced gastric injury of healthy male volunteers was greater in the morning than in the evening. We have also reported that fasting gastric acid secretion rates and gastric retention times for solids were lower in the morning hours of the circadian cycle. We hypothesized, therefore, that defensive factors in the human stomach may exhibit circadian rhythmicity with greater vulnerability to noxious agents during the morning hours. It has also been hypothesized that an antisecretory agent, with reported protective effects, such as Ranitidine, would affect gastric defense mechanisms differently at different times in the circadian cycle. Transmural electrical potential difference (PD) and prostacyclin (PGI2) production by mucosal biopsy specimens were chosen as putative indicators of gastric defensive status. Accordingly, morning (1000) and evening (2200) studies were performed on 10 fasting healthy male subjects with and without oral Ranitidine (150 mg) given 3 hr before oro-gastric intubation for the measurement of PD and the removal of mucosal biopsy samples.(ABSTRACT TRUNCATED AT 250 WORDS)