Histamine was found to be a potent stimulus for acid secretion in 1920. It was then for about 20 years considered to be identical to the antral factor gastrin. It stimulates the parietal cells--the site of acid production--via an H2-receptor in a dynamic manner--that is, there is a continuous turnover of histamine at the receptor site. Cyclic AMP is formed by the receptor stimulation, but how this is coupled to the proton transport is unknown. It could even be possible that the main function of the histamine action is to initiate the drastic morphologic transformation of the parietal cells which occurs in connection with acid secretion. Possible sites for some acid inhibitors are elucidated.