A recent clinical study with a two-year application of the extract ERr 731 from Rheum rhaponticum demonstrated its efficacy and potentially suggested it safety regarding unwanted endometrial side effects. The aim of the present study is to provide experimental proof for the latter observation in a preclinical experimental animal model by assessing dose-dependent effects of ERr 731 - either alone or in combination with estradiol (E2) - on growth and proliferation in the uterus of ovariectomized (ovx) rats. ERr 731 was given in a dose corresponding to human therapeutic application and additionally in three pharmacologically relevant doses. In addition to uterine wet weight, this study examines the effects of ERr 731 on the uterine mRNA expression of the proliferation marker Ki67, proliferating cell nuclear antigen (PCNA), insulin-like growth factor-1 (IGF-1), type 1 IGF receptor (IGF-1R), the two estrogen receptor (ER) subtypes alpha and beta (ERalpha and ERbeta) and the estrogen-responsive gene complement C3 (C3). ERr 731 did neither stimulate an uterotrophic response in the uterotrophic assay with ovx rats nor stimulate or modulate the expression of genes associated with proliferation. In combination with E2, ERr 731 reduced the E2-induced uterine growth stimulation. These observations were further substantiated by the expression pattern of genes related to proliferation control, in view of the fact that the E2-induced elevation of Ki67 mRNA and PCNA protein levels in the uterus were counteracted by simultaneous treatment of the animals with ERr 731. In conclusion, the experimental findings presented here provide further evidence for the safety of ERr 731 towards unwanted uterine and endometrial proliferative events in response to ERr 731 and support observations from recent clinical trials.