The frequency of spontaneously occuring neoplasms in the urogenital system and endocrine organs was examined in two Syrian hamster colonies [Eppley colony (EC); Hannover colony (HC)]. With the exception of a renal adenoma in an EC female, the urinary systems of animals in both colonies were free of neoplastic growths. A single epididymal adenoma of the male fenital system (EC) was found. However, incidences of female genital tract tumors were 10 (EC) and 3.5% (HC) and accounted for 34 and 11%, respectively, of all tumors in females. Among all genital tract tumors, 3 (EC) and 57% (HC) were ovarian, 14% (HC) were in the fallopian tubes, 81 (EC) AND 29% (HC) were uterine, and 16% (EC), vaginal. The overall incidence of endocrine organ tumors was similar in both colonies (EC, 22%; HC, 19%). The distribution of these tumors was as follows: pituitary gland; EC, 3%, HC, 0; thyroid gland: EC, 28%, HC, 31%; parathyroid gland: EC,11%, HC, 18%; adrenal cortex: EC, 42%, HC, 31%; adrenal medulla: EC, 7%, HC, 0; and endocrine pancreas: EC, 10%, HC, 31%. The morphology of these neoplasms is described and the incidence compared with those reported in other colonies.