By means of a groundbased lidar we have measured potassium layer parameters and the temperature structure in the mesopause region during 55 nights in 1999 at Tenerife (28¿N). The potassium density shows a seasonal variation similar to that at midlatitudes. We also study the nocturnal mean temperatures and their seasonal variations and derive new evidence for the global two-level structure of the mesopause. About midsummer we observed the mesopause close to 86 km with about 183 K for a period of four weeks. Transition periods between the regular (high) and the low altitude state of the mesopause were two weeks each.