During high-power, long-pulse transmission from lower-hybrid-range-of-frequency (LHRF) antennas, the waveguide walls outgas as a result of rf-loss-induced heating. If the resulting pressure rise is too high, power transmission will be adversely affected and additional pumping may be required to maintain the pressure at a low enough value. The outgassing rates of waveguides made of various materials (oxygen-free high-conductivity copper, dispersoid copper. copper-coated carbon fiber composite, copper-coated graphite) were measured during rf injection at high power density (50-200 MW/m(2)) for a duration in the range 100-4700 s. The experiments were performed on a test-bed facility equipped with a 3.7 GHz klystron on multiwaveguide (2 to 18) mock-ups. The effect of the main parameters, namely, the waveguide surface temperature and the initial WAR gas loading ("conditioning"), are analyzed in detail. It is concluded that an outgassing rate of 1 X 10(-5) (5 X 10(-5)) Pa m(3) s(-1) m(-2) at 300 degreesC (400 degreesC) can be considered for most materials. The requirement, in terms of additional pumping, for the International Thermonuclear Experimental Reactor LHRF antenna is finally discussed. (C) 2005 American Vacuum Society.