The pharmacological effects of low dose of omeprazole (Om) are not well known. This prompted us to investigate the effects of a 7-day treatment with a low dose of Om, 10 mg/d (Om10), on gastric acid secretion and serum gastrin levels and to compare the results with those obtained with an effective antisecretory dose of 20 mg/d (Om20). Twelve healthy volunteers received randomly and double-blind for three periods of 7 days, separated by at least 7 days, one capsule of placebo (P), Om10, Om20, given daily in the morning, in fasting condition. The last day of each period, 24 h pH was recorded using a glass electrode connected to a Digitrapper (Synectics). At the end of each pH-metry, acid secretion was measured in basal conditions (BAO), after sham-feeding (SAO) and after i.m. injection of 6 micrograms X kg-1 of pentagastrin (PAO). Whatever the threshold pH chosen, there was a statistically significant difference between P and Om10, P and Om20, and Om10 and Om20. Inhibition of acid concentrations was dose-dependent and prolonged, including nocturnal time. However, when considered on an individual basis, five subjects did not respond to Om10. More than 24 h after the last dose of Om has been administered, BAO, SAO and PAO were significantly reduced by either Om10 (respectively -52, -35 and -28 p. cent) and Om20 (respectively -60, -58 and -50 p. cent). Fasting serum gastrin concentration was significantly increased after Om20 treatment but not after Om10. We conclude that treatment with Om10 has a consistent and long lasting inhibitory action on gastric acidity without statistically significant effect on serum gastrin levels. These results suggest that 10 mg Om daily should be sufficient in some duodenal ulcer patients to effectively inhibit gastric acidity specially when long-term treatment seems to be indicated.