An experimental varicocele was created in the adult rat by partial ligation of the left renal vein. There was a significant bilateral elevation of both testicular blood flow and temperature in the varicocele animals (p < 0.01). Mean testicular blood flow for control and varicocele animals was 29.6 ± 1.0 and 39.8 ± 2.0ml./min./100gm tissue, while mean testicular temperature was 34.4 ± 0.1 and 35.3 ± 0.2C, respectively. A left orchiectomy was combined with a left varicocele to determine if the left testis is essential for the right testicular response to a varicocele. Elevation of right testicular blood flow was not altered by left orchiectomy (p < 0.05); however, right testicular temperature was no longer significantly increased. Mean right testicular blood flow and temperature for this group was 39.0 ± 1.5ml./min./100gm tissue and 34.2 ± 0.15C, respectively. A left sympathectomy was combined with a left varicocele to ascertain if the right testicular response to the left varicocele was mediated through a neural pathway. A significant bilateral increase in testicular blood flow was noted with a left sympathectomy alone, and thereby masked the ability to evaluate the right testicular response to the simultaneous left sympathectomy and varicocele.Elevation of right testicular blood flow in response to the left varicocele is independent of the presence of a left testis and any immune response it may stimulate. The role of the sympathetic nervous system as a mediator of the bilateral varicocele effect remains undetermined.