be able to measure the number density of interacting and merging systems at high redshift at a time when the merging rate is thought to peak. Observations using radio telescopes with sufficient sensitivity and sufficient spectral bandwidth can not only determine the statistics of the evolving density of OH masers with time, but they will also select systems of interest at high redshift, which can be further studied with a variety of techniques to determine physical conditions, dynamics and effects of local environment.

For the purposes of estimating the effect of an increased merging rate at early times, the model explored in Fig. \[cosmcompare.fig\] and the top panel of Fig. \[ratecombo.fig\] can be modified by increasing the number density of OH megamasers in proportion to $(1+z)^m$, in keeping with the idea that, locally, OH megamasers are associated with luminous FIR galaxies, which in turn are found to occur in systems with merging and interacting morphologies (Clements et al 1996). For comparison, an extreme example, $m=4.5$ (Lavery et al 1996) is shown in Fig. \[ratecombo.fig\]. The significance of this detection rate is that an existing telescope, such as the Westerbork Synthesis Radio Telescope, which will reach a noise level below 1 