Gastric fluid pH was measured in five adult horses following nasogastric administration of famotidine, 0.5, 1.0, and 2.0 mg/kg bodyweight (bwt); ranitidine, 4.4 and 6.6 mg/kg bwt and an antacid containing magnesium hydroxide (40 mg/ml) and aluminium hydroxide (45 mg/ml), 120 and 180 ml. Fluid was aspirated through a 16 French nasogastric feeding tube at 15 min intervals, and pH was measured using a pH meter. Basal gastric fluid pH was measured at 20 min intervals for 6 h in each horse and, with the exception of two measurements of 4.66 and 4.17, ranged from 1.42 to 2.41, with a mean pH of 1.88 +/- 0.18. The response to both ranitidine and famotidine was variable between horses and between dosages. The mean gastric fluid pH values following administration of a single dose of famotidine or ranitidine were famotidine, 0.5 mg/kg bwt, 3.63 +/- 1.72; famotidine, 1.0 mg/kg bwt, 3.41 +/- 1.40; famotidine, 2.0 mg/kg bwt, 4.18 +/- 2.06; ranitidine, 4.4 mg/kg bwt, 4.07 +/- 1.25; ranitidine 6.6 mg/kg bwt, 5.34 +/- 1.12. Differences in mean gastric fluid pH between famotidine dose levels and between ranitidine dose levels were not significant (P > 0.10). Gastric fluid pH increased to greater than 6 in three horses receiving 0.5 mg/kg bwt, five horses receiving 1.0 mg/kg bwt and three horses receiving 2.0 mg/kg bwt famotidine. Gastric fluid pH increased to greater than 6 in each of the five horses receiving 6.6 mg/kg bwt ranitidine and in four of five horses receiving 4.4 mg/kg bwt ranitidine. During the 6 h post administration, ranitidine tended to increase gastric fluid pH greater than 6 longer (166 +/- 106 mins) than famotidine (98 +/- 110 mins) and this difference was nearly significant (P = 0.054). The response to antacid was variable and consistently short-lived. The mean gastric fluid pH values for 6 h after administration of 120 ml or 180 ml were 2.42 +/- 0.71 and 2.44 +/- 0.47, respectively. Gastric fluid pH increased to greater than 6 in only two horses, for 15 and 30 mins, respectively. The increase in gastric fluid pH was significantly greater for ranitidine (P < 0.001) and famotidine (P < 0.02) than for antacid. The duration of the effect was significantly longer for ranitidine doses (P < 0.001) and nearly so for famotidine doses (P < 0.055) than for antacid.