The composition of extracellular interstitial fluid collected with a push‐pull cannula from the testes of adult rats.

1. A push-pull cannula has been used to obtain extracellular interstitial fluid from the testes of anaesthetized adult rats. 2. Assuming or having demonstrated that appropriate radioactive markers had equilibrated between the vascular and extracellular fluid compartment of the testis, the dilution of the cannula infusate by extracellular interstitial fluid of the testis has been determined. These dilutions have then been used, with the measured concentrations of sodium, potassium, protein and testosterone in the perfusate, to calculate the concentrations of these substances in undiluted extracellular interstitial fluid of the testis. 3. Sodium, potassium and protein levels in testicular interstitial fluid calculated in this way were similar to blood plasma levels. Testosterone concentrations were certainly no greater than in testicular venous blood, and may have been even less. The results with testosterone and potassium contrast with earlier results obtained with less-physiological techniques.