For ten years now, tritoqualine, a histidine decarboxylase inhibitor, has been advocated in a prevention protocol for anaphylactoid reactions occurring during general anaesthesia. The present double-blind study aimed to quantify the effects of the drug on whole blood histamine levels. 44 patients were assigned at random to two different groups, one taking 300 mg t.i.d. tritoqualine for three days, and the other taking a placebo. The histamine levels were measured before the treatment and 12 h after the last dose of the drug after haemolysis of the sample, by the fluorometric technique preceded by column chromatography. In the group taking tritoqualine, the histamine level fell from a mean of 109 +/- 61 to 91 +/- 41 ng X ml-1, whereas it rose from a mean of 92 +/- 55 to 105 +/- 62 ng X ml-1 in the control group taking placebo. These variations were not statistically significant. In both groups were present four volunteers with a history of allergy; their histamine level fell from 107 +/- 35 to 71 +/- 36 ng X ml-1 after tritoqualine intake (p less than 0.05), whereas it rose from 76 +/- 19 to 162 +/- 36 ng X ml-1 after placebo (p less than 0.05). The small differences found in the whole blood histamine level 12 h after the last oral dose of the drug suggested that the present tritoqualine dose regimen was inadequate to achieve the aims of its prescription.