It has recently been shown that ethynyl oestradiol markedly decreases lipase activity in the liver (Valette, Verine, Salers & Boyer, 1977). These observations, along with reports of oestrogen receptors in the liver (Chamness, Costlow & McGuire, 1975) suggest that oestrogens exert considerable influence over the hepatic metabolism of lipids. Previous investigations (DeLorimier, Gordon, Lowe & Carbone, 1965;Gallagher, Mueller & Kappas, 1966) have suggested that the extent of the hepatic changes could be correlated with the molecular nature of the hormonal compound and might depend particularly on the 17\g=a\-alkyl-substitutedgroup in synthetic oestrogens. The influence of the ethynyl function in the 17\g=a\-position of oestradiol-17\g=b\ on the oestrogen-induced changes observed in hepatic lipases and serum levels of triglycerides has therefore been examined.Adult female Sprague-Dawley rats (220-240 g) were allowed free access to food and received oestradiol-17ß or ethynyl oestradiol (6 µg) by s.c. injection in 0-1 ml olive oil daily for 21 days. Controls received olive oil alone. The animals were killed and bled by decapita¬ tion. The liver was immediately excised, rinsed and homogenized at 4°C with three 1 min passes in a glass-Teflon homogenizer in a solution (10 ml/g) containing 0-25 M-sucrose, 2-5% glycerol and 10 mM-EDTA (pH 7-2). The homogenates were centrifuged sequentially at 600 # for 10 min, 10 000 # for 10 min, 25 000 g for 30 min and 100 000 # for 2 h to yield pellets numbered I, II, III and IV respectively. Pellets were resuspended in 50 mM-sodium phosphate buffer (pH 7-2) containing 5% glycerol and 0-1% Triton X-100. The suspensions were sonicated at 4°C (six times for 10 s, microtip, setting 1 with a Branson sonifier model 12) and then centrifuged at 100 000 g for 30 min. The supernatant fraction derived from pellet IV contained 60-80% of both the tri-and monoester lipase activities assayed at alkaline pH in the crude homogenate and served as the source of enzyme. Activity at alkaline pH corresponded to the hepatic lipases specifically sensitive to oestrogens (Valette et al. 1977). Triester lipase was assayed at 37°C in a final volume of 1 ml containing 0-1 ml Tris-HCl, 0-25% defatted albumin, 1 mM-tri-[3H]oleoylglycerol (106 counts/min) at a final pH of 8-0. The assay system for monoester lipase con¬ tained 0-1 mM-Tris-HCl, 0-5% defatted albumin and 1 mM-[3H]oleoylethanol (0-8 IO6 counts/min) in a final volume of 1 ml at 37°C (pH 8-5). In both cases, the release of [3H]oleic acid over the 15 min period of assay was monitored as described previously (Arnaud & Boyer, 1976). One unit of lipase activity corresponded to the release of 1 µ acid/min. Serum levels of triglycérides were determined according to the method of Van Handel «fe Zilversmit (1957).The data are shown in Table 1. Treatment with ethynyl oestradiol reduced the levels of hepatic tri-and monoester lipases by 59 and 22% respectively and these decreases in enzymic activity were associated with a 93% increase in the serum level of triglycér...