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Notes: Abstract Histamine release by drugs used in anaesthesia and surgery has been often demonstrated in human volunteers, but only occasionally in patients. Three questions arose from these studies. (1) Is the incidence of histamine release high in patients during routine anaesthesia and surgery? (2) Can the clinical effects of histamine release in man be prevented by H1+H2-receptor antagonists? (3) Are there any side-effects of such a premedication? These problems were investigated in patients and volunteers by randomized controlled clinical trials using only one of the histamine-liberating drugs in man, the plasma substitute Haemaccel. This drug was chosen because it causes a reproducible histamine release in man and because its mechanism of action in man is largely known. (1) Out of 600 orthopaedic patients 30 (5%) showed anaphylactoid reactions following Haemaccel infusion. 26 of these had a histamine release of more than 1 ng histamine/ml plasma. Using predictive values this gives an efficiency of the test by nearly 98%. (2) In volunteers the combination of an H1-plus H2-receptor antagonist (dimethpyrindene and cimetidine) completely prevented the clinical effects of histamine release by Haemaccel (9 allergoid and anaphylactoid reactions in the control group, none in the H1+H2-group). The incidence of histamine release, however, remained unchanged. (3) The premedication was found to release histamine itself. Cimetidine was effective when given alone but especially in combination with chlorpheniramine (4 events out of 7 applications). The clinical side-effects of these premedication were mild since apparently the free histamine was largely blocked at the receptor sites. It is concluded that premedication with a combination of H1- and H2-receptor antagonists is indicated due to the high incidence of histamine release during anaesthesia and surgery induced by various drugs and treatments. Such premedication is effective but associated with mild side-effects. For this reason more extended clinical trials with dimethypyrindene plus cimetidine in patients are necessary before this premedication can be generally recommended.