The gonadotrophin preparations that have been used previously to study different aspects of testis function are of limited purity. We have, therefore, purified existing gonadotrophin preparations further. The demonstration of their purity and biological activity are reported as well as their suitability for in-vivo use. After separation of the subunits of the intact hormones by high-performance liquid chromatography followed by analytical sodium dodecylsulphate-polyacrylamide gel electrophoresis, no contaminating proteins could be detected in either the FSH or LH preparation. After immunoadsorption, contamination by other pituitary hormones (TSH, prolactin, FSH, LH and GH) was found to be less than 0.002% by weight for all the hormones tested. The biological activity of porcine FSH (pFSH) measured in a Steelman-Pohley assay was 150-170 times more potent than the NIH-FSH-P1 reference preparation. The biopotency of human LH (hLH) in the ovarian ascorbic acid depletion test was measured and appeared to be 8100-8300 IU/mg against the 68/40 International Standard. The dose-dependent effects of pFSH and hLH on testis weight and the number of FSH and LH receptors were measured in immature (22-day-old) hypophysectomized rats treated for 7 consecutive days. Treatment with FSH induced a dose-dependent increase in testis weight (threefold) when compared with the control. The concentration and total number of LH receptors were increased (two- to sixfold) in a dose-dependent manner. The number of FSH receptors per testis increased while the number of FSH receptors per mg of protein remained unchanged. Administration of human LH to immature hypophysectomized rats had no effect on either LH or FSH receptors, regardless of the dose administered.(ABSTRACT TRUNCATED AT 250 WORDS)