Abstract(1) Gas exchange measurements onRhytidiadelphus loreusandAnomodon viticulosusfollowing re-moistening after a period of desiccation showed an initial stimulation of respiration, followed immediately by progressive recovery of photosynthesis within the first 1–6 h. A. viticulosus after 48 h dry reached the compensation point within a few minutes of re-moistening; R. lore us after 7 days dry reached the compensation point after about 4 hours.(2)Hylocomium splendens,R. loreusand Tortula ruraliformiswere subjected to the following patterns of alternating wet and dry periods over a period of 18 weeks: continuous dry, 1 day wet/6 days dry, 6 days wetll day dry, 1 day wet/1 day dry, 7 days wet/7 days dry, continuous wet.(3) OnlyT. ruraliformisstill attained a positive net assimilation rate after the continuous dry treatment for the duration of the experiment. All three species retained a positive net assimilation rate throughout after all the remaining treatments.(4) A moist period of 24 h allowed substantially complete recovery from the after-effects of preceding desiccation.(5) Experiments in which two dry periods were separated by moist periods of 6 h, 24 hand 7 days showed that recovery inH. splendensandR. loreuswas incomplete in 6 h but confirmed that it was complete in 24 h.(6)H. splendensshowed constant or slightly increased desiccation-tolerance in successive dry periods.R. loreusshowed a well marked increase in tolerance (hardening) following a previous dry/wet cycle.(7) The effect of intermittent desiccation can be predicted approximately from the successive effect of the individual dry periods.(8) It is doubtful whether there is any specific deleterious effect of drying and re-moistening; any such effect that does exist can only be small.