Testosterone was measured by radioimmunoassay in interstitial fluid, 'free-flow' seminiferous tubular fluid, obtained by micropuncture, and rete testis fluid from intact adult anaesthetized rats. Under non-stimulated conditions the concentration of testosterone in interstitial fluid was below the limit of detection in two rats and achieved a mean level of 150 +/- 27 (S.E.M.) ng/ml in the remaining 17 determinations. The testosterone concentration of the seminiferous tubular fluid was below the limit of detection in two rats, and had a mean level in the remaining 15 determinations of 91 +/- 14 ng/ml, which is significantly lower (P less than 0-02) than that in interstitial fluid. The mean ratio of seminiferous tubular:interstitial fluid testosterone concentration calculated in 14 rats was 0-94 +/- 0.24. This ratio was less than unity whenever the interstitial fluid testosterone concentration was more than 50 ng/ml, whereas in all animals with interstitial fluid testosterone of 50 ng/ml, or less, the ratio was greater than or equal to one. The mean testosterone concentration of rete testis fluid in 32 samples was 33 +/- 3 ng/ml. After HCG stimulation in 12 rats, testosterone concentration in interstitial fluid increased to a mean value of 660 +/- 83 ng/ml, and in seminiferous tubular fluid to 460 +/- 44 ng/ml; the difference between the two was significant (P less than 0-05). These results are discussed in relation to the presumed dilution of seminiferous tubular fluid in rete testis fluid and the role of androgen-binding proteins in the transport of steroids.