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Nine blood samples were taken at 30-min intervals from 36 Landrace x Large White boars at each of eight ages (42, 56, 70, 84, 98, 112, 126 and 140 d). Serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone (T) and estradiol- 17β (E-2) were quantified by radioimmunoassay procedures. The maximum concentration of LH and the age at maximum concentration were predicted for each boar. Variability of LH samples was described for each boar by the pooled within age variance among LH samples and by the number of LH peaks. Measurements of testicular development taken at 140 d of age included: in situ testis width and length, excised testis weights and histological traits of excised testes (seminiferous tubule diameter, percentage of tubules with a lumen and percentage of tubules with active spermatogenesis). Pooled within line correlations were calculated with data from boars selected for either high or low testis weight. Correlations among the testicular traits ranged from .45 to .88. Luteinizing hormone concentration (mean over all ages) was related to measures of testicular development (r = .24 to .49). Concentrations of LH from 42 to 84 d of age were more highly correlated with testicular traits than were the concentrations from 98 to 140 d. Boars with larger, more mature testes tended to have higher maximum concentrations of LH (r = .19 to .42) and younger age at maximum concentration (r = -.12 to -.26). Testicular traits were correlated with the number of LH peaks (r = .31 to .43) but not with LH sample variance (r = -.14 to -.01). Concentration of FSH was not related (r = -.20 to .19) to measures of testicular development. Testicular traits were more highly correlated with E-2 concentration (r = .22 to .41) than with T concentration (r = .12 to .33). Correlations between gonadotropin and steroid concentrations were small and nonsignificant.