The effect of growth hormone (GH) on arylesterase (AE), one of the activities of paraoxonase, has never been studied. The aims of the present study in mice were: (a) to compare the effect of age and sex on serum lipid and lipoprotein levels after consumption of lactalbumin- v. chow-based diets and (b) to study the effect of GH administration, age and sex on serum AE activity, lipid and lipoprotein and body fat levels in mice fed a lactalbumin diet. Seventy-two mice were divided into three age- and sex-matched experimental groups: (1) control chow (CC), (2) non-GH lactalbumin (NGL) and (3) GH-treated lactalbumin (GL) mice. Lactalbumin increased total cholesterol, (LDL+VLDL)-cholesterol and TAG and diminished HDL-cholesterol in all animals (P < 0·05). In comparison with their NGL counterparts, old GL males presented lower total cholesterol (15 %) and (LDL+VLDL)-cholesterol (17 %) levels (P < 0·05), whereas values of the same parameters were higher in adult GL males (P < 0·05) (22 and 23 %, respectively). Adult GL females displayed higher serum HDL-cholesterol concentrations (26 %) (P < 0·05) than adult NGL females. AE activity was lower in old GL females (78 %) and old GL males (20 %) (P < 0·05), but higher in adult GL males (100 %) (P < 0·01). GH, that was inversely related to food intake, decreased abdominal and gonadal fat in all mice (P < 0·05). To conclude, lactalbumin induced an atherogenic lipoprotein profile in NGL mice that was reverted by GH, preferentially in old males, suggesting that GH therapy will be more effective in aged men. The present results suggest that AE activity was age-, sex- and body fat level-dependent and that it diminished as a consequence of improved antioxidant status.