Compounds of formula (I), wherein: the symbol ---- represents a single or a double bond; R is a hydrogen atom or a C.sub.1 -C.sub.4 alkyl group; R.sub.f and R'.sub.f, each independently, are C.sub.1 -C.sub.4 alkyl groups substituted by one or more fluorine atoms; and R.sub.1 and R.sub.2, each independently, are selected from: a hydrogen atom; a phenyl group; a C.sub.1 -C.sub.4 alkyl group unsubstituted or substituted by one or more fluorine atoms; a halogen atom; a cyano (CN) group; a group OR.sub.4, wherein R.sub.4 is a hydrogen atom or a C.sub.1 -C.sub.4 alkyl group; a group SR.sub.5, wherein R.sub.5 is a hydrogen atom or a C.sub.1 -C.sub.4 alkyl group; and a group COR.sub.6, wherein R.sub.6 is a group OR.sub.4 in which R.sub.4 is as defined above or a C.sub.1 -C.sub.4 alkyl group unsubstituted or substituted by one or more fluorine atoms. They are useful as testosterone 5a-reductase inhibitors.