1. 19-Nor-17alpha-pregna-1,3,5(10)-trien-20-yne-3,17-diol (ethynyloestradiol) or 17beta-hydroxy-19-nor-17alpha-pregn-4-en-20-yn-3-one (norethindrone) but not 17alpha-ethyl-17beta-hydroxy-19-norandrost-4-en-3-one (norethandrolone) caused a time-dependent loss of cytochrome P-450 when incubated in vitro with rat liver microsomal fractions and NADPH-generating systems. 2. The enzyme system catalysing the norethindrone-mediated loss of cytochrome P-450 had many characteristics of the microsomal mixed-function oxidases. It required NADPH and air, and was inhibited by Co. However, it was unaffected by 1 mM-compound SKF 525A. 3. In microsomal fractions from phenobarbitone-pretreated rats the norethindrone-mediated loss of cytochrome P-450 was increased relative to controls. The norethindrone-mediated cytochrome P-450 loss was less pronounced when the animals were pretreated with 3beta-hydroxy-pregn-5-en-2-one 16alpha-carbonitrile (pregnenolone 16alpha-carbonitrile). Pretreatment with 3-methylcholanthrene rendered the animals resistant to the norethindrone effect. 4. Administration in vivo [100mg/kg, intraperitoneally] of norethindrone or ethinyl oestradiol also produced a time-dependent loss of liver cytochrome P-450. Norethandrolone had a similar, though much less-marked, effect. All three steroids lead to an induction of 5-aminolaevulinate synthase and an accumulation of porphyrins in the liver. 5. The loss of cytochrome P-450 and the accumulation of porphyrins in the liver 2 h after the administration of norethindrone to female rats was similar to that seen in males. 6. Rats pretreated with phenobarbitone and given norethindrone or ethynyloestradiol (100mg/kg, intraperitoneally) formed green pigments in their livers. These had characteristics similar to the green pigments produced in the livers of rats after the administration of 2-allyl-2-isopropylacetamide. No green pigments could be extracted from the livers of control rats or those given norethandrolone, oestradiol or progesterone.