Possible effects of weather modification by cloud seeding to release rain are examined on the basis of extensive published information under the headings, effect on primary productivity, factors affecting grassland response (nutrients, rainfall pattern, genetic limitation), effect on range composition (species and genotypic changes), effect on secondary productivity, and other potential effects (plant diseases, parasites, run-off). It is concluded that added water in the short run will increase primary production in yr of low precipitation during the growing season, opt. timing being in spring or early summer, but that without fertilization forage gains will be slight. Long-term incremental forage production will be governed by the effect of added water on nutrient cycling rates and community composition will change, depending on the timing of the additional rainfall. In the grasslands of the N. Great Plains, increased forage in the absence of increased N, may have a neutral or negative effect on livestock wt. gains.