The variations in blood pressure, heart rate and plasma histamine levels were monitored for 10 min in 40 patients at risk of releasing histamine after they had been given 2.5 mg X kg-1 propofol in oil emulsion (n = 20) or 7 mg X kg-1 thiopentone (n = 20). The measurements were carried out before anaesthesia, and 1, 2, 3, 5, 7 and 10 min after injecting the drug. All the patients were premedicated with a specific anti-H1 antagonist only (terfenadine). Histamine was measured by radio-immunology (Immunotech). Two patients from the propofol group and three from the thiopentone group showed minor signs, that may have been linked to a release of histamine. Blood pressure fell in both groups by about 20% (p less than 0.001) within the first minute, becoming stable afterwards. The heart rate did not change significantly. Plasma histamine levels did not change in either group, nor in those patients who had presented a local erythema. Propofol did not seem to induce non-specific histamine release when injected over a period of 60 s in patients at risk of doing so.