Gain degradation measurements have been carried out on five 419BL and two 318BL Mullard lead-type channel electron multipliers, over periods up to several thousand hours, at a pressure of less than 10 ntorr. The devices showed reductions of about one order of magnitude on their initial gain values. The reduction occurred in 10 h at an output count rate of 2 × 10⁵ counts s⁻¹ and several thousand hours at 2 × 10² counts s⁻¹. At the high output count rate, the initial gain was found to be depressed by over an order of magnitude on the nominal value of 10⁸.