Evening dosing has become standard for H2 receptor antagonists, because available agents inhibit nocturnal basal acid secretion more effectively than daytime stimulated secretion. We studied the optimal time of administration of a new high affinity long acting H2 receptor antagonist, SKF 94482, for the suppression of intragastric acidity using intragastric telemetry. Sixteen healthy subjects received SKF 94482 200 mg or placebo at 07:30, 17:30, and 21:30 h during four separate studies. Time (h) above pH 4 was (mean (SD] 1.1 (1.2) on placebo, 7.8 (5.0) on SKF 94482 given at 07:30, 5.75 (3.6) on SKF 94482 given at 17:30, and 6.1 (2.9) when given at 21:30. All treatment regimens were effective in increasing time above pH 4 (p less than 0.01). The efficacy of the morning dose of SKF 94482 indicates that the best time to give H2 receptor antagonists depends on their pharmacological properties.