Five triphenylethylenes, a triphenylethane and a triphenylethanol, carrying methyl substituents ortho to one or both of the ring oxygen functions, have been examined for oestrogenic, and antioestrogenic activity in mice, and three of the compounds, alpha-[4-(beta-diethylaminoethoxy)-3, 5-xylyl]-alpha-phenyl-beta-4-methoxyphenyl-ethanol (IV), alpha'-[4-(beta-diethylaminoethoxy)-3, 5-xylyl]-4-methoxy-bibenzyl (V) and alpha'-[4-(beta-diethylaminoethoxy)-3, 5-xylyl]-4-methoxy-stilbene (VI), were tested for their effects on fertility in mice. 2 Orthomethylation reduces oestrogenic and/or anti-oestrogenic activity compared with the reported activities of non-methylated analogues. 3 The anti-oestrogenic ethamoxytriphetol (MER 25) reduced fertility in mice whereas its inactive dimethylated analogue (IV) was ineffective. The weakly active anti-oestrogens, V and VI, did not affect fertility in mice.