The stratospheric and mesospheric sounder (s.a.m.s.) instrument was launched on the Nimbus G satellite on 24 October 1978. It is designed to measure temperature and concentration profiles of various gases in the height range 20-100 km by detecting either their thermal emission or, in some cases, resonant scattering of sunlight. The gases selected, CO 2 , CO, CH 4 , NO, N 2 O and H 2 O, significantly affect the upper atmosphere energy budget by their influence on the concentration of the primary sunlight absorber, ozone. This influence is disproportionate to their own concentration because of the existence of 'catalytic cycles’ which destroy ozone while regenerating the catalyst. A description of the instrument, its principles of operation and some of the methods of retrieval used is presented, together with some preliminary results from the first 3 months of operations.