Activation by and desensitization to signals originating from the environment is common in all organisms, ranging from prokaryotes to higher eukaryotes. Recently, a number of novel mechanisms involved in activation and desensitization have been elucidated. The similarities of the molecular mechanisms of activation and desensitization in the various systems are striking. In the first book (1987) on this topic, the functional and structural similarities of the various mechanisms to attenuate the effect of an outward signal were emphasized. Justification to have a second book three years later is the explosive growth of information in the field. More components involved in signal transfer have been discovered and also that several more G-proteins appear to play a role during this process. The interest in signal attenuation concurs with these developments. The emphasis in this book is on the common signal transfer elements responsible for activation and desensitization in animals, plants and microorganisms.

All organisms adapt, or at least try to adapt, to informa- tion or signals arising from their surrounding environment. This can affect the behaviour of the entire organism or the functioning of discrete organs or areas within an organism, depending upon its degree of complexity. This workshop was set up with the aim of encouraging discussions and collaboration between investiga- tors studying a wide spectrum of signal transduction/sensing events occurring at the cell surface in systems where the sig- nalling mechanisms became attenuated or desensitized after their initial stimulation/activation. To this end a wide range of sys- tems were dealt with ranging from simple organisms to higher eUkaryotes. Desensitization has been of much interest for some time. Recently, however, a number of novel mechanisms have been elucidated and, in order to capture the present excitement in this field, the participants have been encouraged to produce articles which would present an overview of their field and research effort which, we hope, will be useful to the int- ested non-speciaJist and specialist alike. A brief transcript or the ensuing discussions has also been included ror clarifi- cation and to highlight both points of comparison between sys- tems and of particular interest.