 Dear students, in this topic we shall discuss the intracellular signaling mechanism of lipid insoluble hormones. The lipid insoluble or water soluble hormones do not penetrate the cell. Their receptors are present on the cell surface membrane. So the hormone receptor complexes are formed on the cell membrane of target cell. The binding of hormone to its receptor results in the activation of many cellular proteins. This results in a cascade of chemical reactions which involve a number of enzymes and proteins as a result a second messenger is formed. It is this second messenger through which the hormonal action propagates further. The second messenger subsequently causes intracellular effects that transduce the extracellular signal received from the hormone and transduce this signal into the specific intracellular response. Dear students, the responses of water soluble hormones vary. There is quite a large diversity of such hormonal actions which are exhibited by lipid insoluble hydrophilic hormones. These responses include the activation of some enzymes, a change in the uptake or secretion of specific molecules, rearrangement of cytoskeleton of the cell and movement of certain proteins into the nucleus which can alter the transcription of genes. This is the work of Mukhliw. Dear students, the second messengers which are involved in signal transduction pathways of water soluble hormones are of three types. These second messengers may be cyclic nucleotide monophosphates for example CAMP and CGMP that is cyclic adenosine monophosphate and cyclic guanosine monophosphate. The second type of second messengers produced in these pathways are the inositol phospholipids for example inositol trisphosphate and diacylglasrol dag. The third type of second messengers produced in the pathways of lipid insoluble hormones are the calcium ions and the protein called modulin which is associated with these calcium ions.