Abstract: Making progress in the study of hormone-sensitive signaling systems in vertebrates and human requires a better understanding of how chemosignaling systems in lower eukaryotes originated and how molecular mechanisms of signal transduction via these systems function. This review is devoted to the structural-functional organization of chemosignaling systems and their components in unicellular organisms such as Dictyostelium discoideum, yeasts and related fungi, flagellates, and ciliates. The attention is focused on receptors of the serpentine type, heterotrimeric GTP-binding proteins and adenylyl and guanylyl cyclases, generators of cAMP and cGMP, present in various forms in a majority of eukaryotic signaling systems coupled with G proteins. Signaling systems involving the receptor component not coupled with G proteins, the receptor forms of adenylyl and guanylyl cyclases of Trypanosoma and ciliates, in particular, are also analyzed. A comparison of signal transduction systems of lower and higher eukaryotes revealed a number of peculiarities and similarities between them. The problem of evolution of chemosignaling systems in lower eukaryotes is viewed through the authors' hypothesis about the prokaryotic genesis of the systems.