M.Sc. (Zoology) / The histomorphology of the ovaries of Cyprinus carpio L., as well as the histomorphological changes that occur in these ovaries during the breeding cycle, were investigated. Concurrently, the blood plasma and ovarian cytosol were analized for the presence of gonadotropin (GtH), FSH, LH, Oestriol, Cortisol and Progesterone, with the aid of Test Kits which are commonly used for determining the levels of these hormones in human blood plasma. C. carpio possesses cystovaria which are continuous with the oviducts which unite before opening to the exterior via a gonopore. The results clearly show that C. carpio has a four season breeding cycle and that egg laying occurs during Spring and Summer. Furthermore morphological changes of the ovary (which occurred during the breeding cycle) as well as the oocyte developmental stages, closely resemble those respectively described for teleosts in general by Kesteven (1960) and Malhotra et al., (1978). The endocrinological tests clearly indicated the presence of GtH, FSH, LH, Oestriol and Cortisol, while all the tests for Progesterone were negative. A comparison between the presence of the abovementioned hormones and the histomorphological development of the ovaries during the breeding cycle indicate that the endocrinological control mechanisms are similar to those which have been described for other teleosts. However, a clear distinction was found between FSH and LH which is a possible indication that C. carpio does not possess a single gonadotropin, but rather that FSH and LH have evolved in this species.