Endogenous immunoreactive luteinizing hormone - releasing hormone (LRH) in plasma was determined in 6 male and 7 female patients with chronic renal failure before and during haemodialysis. Basal plasma LRH levels ranged from 5.8 to 23.0 pg/ml and, in 11 out of 13 patients, were above the levels seen in healthy subjects (less than 7 pg/ml for men and less than 8 pg/ml for women). This immunoreactivity was eluted iun identical fractions with synthetic LRH and plasma extracts from climacteric women on Sephadex G-25 chromatography, and the dilution gave a displacement curve parallel to the standard. Within 5 h of haemodialysis, these high LRH levels declined into the normal range. The concentrations of LH in plasma in these patients were also elevated, but those of testosterone in male patients were decreased. These results suggest 1) that elevated plasma LRH reflects decreased feedback inhibition by primary gonadal failure and might in turn be responsible at least in part for high concentrations of plasma LH in chronic renal failure, and 2) that plasma LRH is mainly not bound to plasma proteins.