A number of indole derived melatonin analogues have been prepared with the C-3 amidoethane side chain partially constrained by incorporation in a ring. The biological achvity has been correlated with the conformation of the "side chain", the nature of the N-acylating group, and the spatial distance between the methoxyl and amide functions.The pineal hormone melatonin (la) plays a major role in the regulation of seasonal cycles and the control of circadran rhythms'** and has been the focus of considerable clinical interest3 As part of a programme to map the melatonrn receptor by examrning the spahal and electronrc restnctions that it imposes on melatonin anafogues for them stall to act as agonrsts4 we have synthesised a number of mdoles annelated on the [b] face of the pyrrole moiety. Such compounds have constrained or partially constrained conformations of the C-3 amidoethane side chain. We chose indole as the basis for our model system in order to minimrse deviations from the natural melatonin structure, since we were also concerned with the relative spatial arrangement of this side chain and the methoxyl group at a position equrvalent to C-5 in melatonin We now report the preparation and biological activity of a number of N-acyl-9amino-1,2.3,4-tetrahydrocarbabazoles and N-acyl-4-ammomethyl-9-methyl-l ,2,3,4-tetrahydrocarbazoles.5