Serotonin (5-hydroxy-tryptamine, 5-HT) is one of the major monoamines in the brain. It is involved in global functions such learning, mood etc. Recently, its neuroprotective function was also revealed. Serotonin is also found in the retina, which is known to be part of brain. A subgroup of amacrine cells (AC) is considered to be the sole retinal source of 5-HT, although some bipolar cells (BC) accumulate and release him (Wilhelm et al, 1993; Shütte, 1994; Ghai et al, 2009). Serotonin receptors are poorly studied and the data obtained are too contradictory. That’s why the aim of present investigation was to study the distribution of serotonin synthesizing neurons and serotonin receptors 5-HT1A, 5-HT2A, 5-HT3A, 5-HT4, 5-HT5A, 5-HT6 and 5-HT7 in the retina of frog (Rana ridibunda) and turtle (Emys orbicularis), two lower vertebrates species with mixed and predominantly cone type of retina resp. All procedures with a frog and a turtle were performed in accordance with the guidelines of Ethical commission of Medical University Sofia, Bulgaria and the EU legislation. The animals were deeply anesthetized in water containing tricain-methansulphonate (1000 mg/L) and decapitated. The retinas were further processed by the indirect immunofluorescence method, as described earlier (Vitanova et al, 2001). More than ten antibodies directed to the key enzyme in serotonin synthesis – tryptophan hydroxylase (TRH), and to the above mentioned 5-HT receptors were used. The results showed strong TRH immunoreactivity in frog and turtle retinas. The AC and BC perikarya, as well as their final endings in outer (OPL) and inner (IPL) plexiform layers were labeled. A lot of varicosities were seen along the AC processes, running in parallel in S1 and S3 sublayers of IPL, while the BC axons usually had hemi-ring shaped ramifications in S1, S3 and S5. Single horizontal and ganglion cells were also labeled, as well as bundle of parallel axons in the proximal retina. All 5-HT receptors studied were also very well expressed. In frog, numerous labels were found in both plexiform layers that show synaptic localization of serotonin receptors. In turtle, labels in the nuclear layers prevailed that was interpreted as a sign for extra-synaptic effects of serotonin. The 5-HT6 and 5-HT7 receptors, the data for which (as we know) are the first immunocytochemical data seems to be mainly involved in the distal retina information processing. All the results give good ground to affirm that the serotonin retinal sources are more numerous than was previously thought, which is in agreement with recent studies on the TRH mRNA expression in retina (Cornide-Petronio et al, 2013, 2015). The numerous receptors, distributed widely in retinal pool show that serotonin fulfills multiple functions, serving both as neurotransmitter and neuromodulator.