Serum gastrin levels have been studied in 70 patients with chronically reduced glomerular filtration rate (GFR) as estimated by 51Cr-EDTA clearance, creatinine, and beta2-microglobulin values. A strong dependence upon GFR was found, although the correlation between gastrin levels and GFR was not as high as that between beta2-microglobulin and GFR, indicating the existence of extrarenal factors regulating the levels of circulating gastrin. In a separate group of 31 patients on maintenance dialysis the mean gastrin level was 65.9 pmol/l--that is, a fourfold increase compared to healthy subjects. Three of the uraemic patients had pronounced rises in serum gastrin in the range 800-1800 pmol/l. Finally, the influence of acute alterations of kidney function on serum gastrin was studied in 11 patients undergoing renal transplantation. In addition to a GFR dependence the results indicate the existence of feedback mechanisms in gastrin homeostasis. Although the clinical importance of the increased gastrin levels in renal failure is unknown, hypergastrinaemia occurs with sufficient frequency to be involved in upper gastrointestinal complications of uraemic patients.