Rat testes fixed by vascular perfusion were examined by scanning and transmission electron microscopy. This revealed a complex organization of the interstitial tissue and an extensive "testicular lymphatic space" composed of continuous "peritubular lymphatic sinusoids." No cellular connections exist between the interstitium and the tubules. Each sinusoid completely surrounds an individual seminiferous tubule, and adjacent sinusoids communicate freely through fenestrae in the interstitium. Thus, material must enter the lymph to gain access to the tubules. The sinusoids are delimited by separate endothelial cell layers over the tubules and interstitium. The layer over the interstitium is discontinuous in specific areas according to the geometry of the tubule packing. Interstitial tissue located in the flattened biconcave interstices between two adjacent tubules is consistently covered with endothelial cells, while that located in the open, triangular interstices between three or more tubules consistently lacks an endothelium. Most of the Leydig cells are located in the "open interstitium" and are directly exposed to the lymph. Physiological implications of the "testicular lymphatic space" and additional features of the interstitium are discussed.