The growth response of the remaining intact testis or testis fragments to partial castration was studied as a function of the duration of the postoperative period, the amount of testis mass excised, as well as the functional state of the testes at the time of operation. Excision of about 90% of the testis mass caused a growth response that increased from slight after eight weeks to pronounced after 14 weeks. After 14 weeks the growth response was slight tounilateral excision of 75% of a testie and pronounced to bilateral excision of 75% of each testis. Subtotal castration caused formation of new seminiferous tubules within the remaining testis tissue when the operation was performed early in the annual testis cycle, whereas the growth response late in the cycle was primarily caused by increased spermatogenetic activity within existing tubules. Partial castration stimulated oocyte formation within fragmented testes, but not in a remaining intact testis. Oocyte formation within a testis fragment was independent of the presence of the Bidder's organ.