1. We determined plasma levels of histamine in uraemic patients and examined their correlation with the presence of pruritus. 2. In 27 patients with chronic renal failure, plasma histamine levels were analysed by radioimmunoassay and were compared with those of 40 healthy adult subjects. The control population showed plasma histamine concentrations of 185 +/- 33 pg/ml, which were significantly lower than those of the patients with renal insufficiency. The highest levels (552 +/- 116 pg of histamine/ml) were found in 16 patients with chronic renal failure (mean serum creatinine 5.1 +/- 1.0 mg/dl) and severe itching. 3. Twelve patients with pronounced pruritus who were on maintenance haemodialysis (serum creatinine 9.2 +/- 1.2 mg/dl) had a mean plasma histamine concentration of 515 +/- 81 pg/ml. Fifteen patients on regular haemodialysis (serum creatinine 9.0 +/- 1.5 mg/dl) and who experienced itching had plasma histamine levels (322 +/- 40 pg/ml) which were significantly lower (P less than 0.01) than those of the patients with pruritus but which were elevated compared with those of the control population (P less than 0.01). 4. No correlation could be found between increased plasma histamine levels and the type of dialysis membrane used or the method of sterilization of the membrane. 5. Haemodialysis alone did not reduce plasma histamine concentrations, although high concentrations could be detected in the ultrafiltrate. In six patients a rapid decrease in plasma histamine concentration from 565 +/- 134 pg/ml to within the normal range could be detected after 60 min of combined haemodialysis and haemoperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)