A 50 m(2) RPC carpet was operated at the YanBaJin Cosmic Ray Laboratory (Ti
bet) located 4300 m a.s.l. The performance of RPCs in detecting Extensive A
ir Showers was studied. Efficiency and time-resolution measurements at the
pressure and temperature conditions typical of high mountain laboratories,
are reported. (C) 2000 Elsevier Science B.V. All rights reserved.