The growth-inhibiting effects of the long acting somatostatin analogue Sandostatin on the transplanted Dunning R3327-H androgen-sensitive rat prostate tumor were investigated. Recipient animals were male Copenhagen × Fischer F1 rats (N = 36). When mean tumor volume reached 700 mm3 (20 weeks following transplantation), the rats were divided into four groups: control; Sandostatin (100 µg/kg s.c. twice a day); castrate; castrate/Sandostatin. Tumor size was assessed by magnetic resonance imaging 21, 42, 63, 105, and 138 days subsequently. Administration of Sandostatin was interrupted between days 43 and 62. As assessed by transplant volume, Sandostatin caused a moderate (up to 50%) but highly significant (P < 0.001) suppression of tumor growth in the intact rats; the effect was reversed when drug administration was stopped. In the castrates, in which tumor growth was markedly less than in intact rats, no significant effect of Sandostatin was seen. Analysis of the tumor growth rate demonstrated that Sandostatin led to a 19% reduction (P < 0.05) in growth rate in intact rats and a 9% decrease (not significant) in castrates. These findings extend previous reports of partial suppression of various types of tumors in vivo with Sandostatin and other somatostatin analogues. Their relevance with regard to the possible use of Sandostatin in the treatment of prostatic carcinoma in humans is discussed.