Summary. The antifertility effects of two non-steroidal compounds, clomiphene and a diphenylindene derivative (U-11555A), were studied in the mouse, using two different methods. First, the effects of various concentrations of the compounds on the in-vitro development of two-cell mouse embryos into blastocysts were investigated. Second, the oral doses of the compounds which were necessary to block pregnancy in mice were determined. It was found that the inhibitory doses for mice are somewhat higher than for rats. Although it is impossible to make direct comparisons between the effective in-vivo and in-vitro doses, the inhibitory concentrations seem to be of nearly the same order in both cases.

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