SummaryPaper chromatography has revealed a proteinuria and generalized aminoaciduria in young mice homozygous for the mutationbrain herniaat a time prior to the development of severe polycystic disease. As the polycystic disease becomes established, the urine becomes more dilute than normal.The urine of very young mice of both mutant and normal genotypes has proved to be excellent material for the chromatographic analysis of amino acids.The author acknowledges with thanks many helpful discussions with Prof. L. C. Dunn during the course of this project.