The contribution of the kidney to the metabolism of prolactin has not yet been established. In the present study, urinary prolactin concentrations in 30 children with renal disease were measured by a newly devised, highly sensitive, time-resolved immunofluorometric assay. Median prolactin concentrations in the urine of children with renal disease, were 1.86 pmol/L-1 of urine (range: 0.17-546.31 pmol/L-1. By stepwise regression analysis, change in urinary prolactin levels as a function of the urinary beta 2-microglobulin concentrations was detected. These results indicate that prolactin urinary excretion in children with renal disease is dependent on the renal proximal tubular function.