The ultrastructure of the main cellular components of testicular interstitial spaces in three Anuran species was studied during two short periods of the seasonal cycle and after treatment with 5- and 6-hydroxydopamine (5- and 6-OHDA). In early December when seminiferous tubules are completely filled with resting spermatozoa in Rana, Leydig cells display a well developed smooth- and rough-surfaced ER, numerous granular vesicles being untypical of Mammalian Leeydig cells, metochondria with tubular cristae and only few lipid droplets. In late April shortly after spermiation has occurred in most animals studied Leydig cell exhibit a low degree of activity as shown by the presence of numerous large lipid inclusions, reduced amounts of ER and granular vesicles, but increased numbers of dense bodies. In addition, a few rather undifferentiated cells are observed. In principle, the same differences between winter and late spring Leydig cells are seen in Bufo, although the smooth-surfaced ER which is partly arranged in whorls is much more pronounced in this species whereas granular vesicles are lacking. In December intertubular spaces display a few mitotic figures of Leydig cells. In Xenopus no obvious seasonal changes are observed in Leydig cells. As in Rana, there are numerous granular vesicles. The agranular ER is moderately developed. Intratesticular seminal excretory ducts consist in all three species studied of very low differentiated cells being almost completely filled with filaments (equivalent-100 A). Seasonal changes do not seem to occur. Morphological equivalents for secretory or resorptive processes are neither observed in early December nor in late April around the time of spermiation (in Rana and Bufo). At the connections site of seminiferous tubules and excretory ducts a basal laminal labyrinth is commonly observed. In Xenopus smooth muscle cells from an incomplete sheath around seminiferous tubules and are also uniformly distributed in the intertubular spaces without any obvious relationships to vessels. Rana and Bufo lack a peritubular contractile cell layer. After application of 5- and 6-OHDA (5-OHDA: three injections of 200 mg/kg at 12 hr intervals; 6-OHDA: three doses of 100-150 mg/kg on three consecutive days) cytoplasmic inclusion bodies with lamellar and crystalloid internal patterns develop in Leydig cells from lysosome-like structures. In addition, formation of dense bodies from mitochondria is seen in a few instances. The significance of these alterations is discussed.