Disturbances of prolactin secretion occur both in the chronic renal failure and in diabetes mellitus. The study aimed to investigate if the diabetic nephropathy as a cause of chronic renal failure disturbs prolactin secretion. The study was conducted in 5 groups of patients: group I-12 patients with IDDM without diabetic nephropathy; group II-12 patients with IDDM with diabetic nephropathy treated conservatively; group III-16 patients with chronic renal failure of non-diabetic origin; group IV-12 patients with IDDM with end stage renal failure in the course of diabetic nephropathy treated with haemodialysis; group V-16 patients with end stage renal failure of non diabetic origin treated with haemodialysis. 12 healthy subjects served as the control group. In all investigated groups as well as in the control group the TRH test was performed. The mean serum prolactin concentration was estimated in the investigated groups just before the intravenous TRH injection and then after 15, 30, 45, 60 and 120 minutes. The mean area over the basic value (AOBV) of prolactin was also assessed. The patients with IDDM without diabetic nephropathy did not differ from healthy subjects both in the basic and TRH induced prolactin secretion. Basic and TRH induced prolactin secretion in patients with diabetic nephropathy both conservatively treated and treated with haemodialysis were lower than in patients with the same stage of chronic renal failure of non-diabetic origin.