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WILLOUGHBY, D. S., and L. TAYLOR. Effects of Sequential Bouts of Resistance Exercise on Androgen Receptor Expression. Med. Sci. Sports Exerc., Vol. 36, No. 9, pp. 1499–1506, 2004.Increased serum testosterone (TST) occurs in response to resistance exercise and is associated with increased muscle mass. However, the effects of elevated TST and sequential resistance exercise bouts on androgen receptor (AR) expression in humans are not well known. This study examined three sequential bouts of heavy-resistance exercise on serum total TST, sex hormone-binding globulin (SHBG) and free androgen index (FAI), skeletal muscle AR mRNA and protein expression, and myofibrillar protein content.Eighteen untrained males were randomly assigned to either a resistance-training [RST (N = 9)] or control group [CON (N = 9)]. RST performed three lower-body resistance exercise bouts, each separated by 48 h. At each exercise bout, RST performed three sets of 8–10 repetitions at 75–80% one-repetition maximum using the squat, leg press, and leg extension exercises, respectively, whereas CON performed no resistance exercise. Muscle biopsies were obtained immediately before the first exercise bout and 48 h after each of the three bouts, whereas blood samples were obtained immediately before, immediately after, and 30 min after each bout. Data were analyzed with two-way ANOVA and bivariate correlations.Serum TST and FAI were significantly increased after each exercise bout (P < 0.05); however, there were no significant changes for SHBG. AR mRNA and protein were significantly increased (P < 0.05) after the second and third exercise bouts, respectively, and were significantly correlated to TST and FAI (P < 0.05). Myofibrillar protein increased after the third bout (P < 0.05).Three sequential bouts of heavy resistance exercise increases serum TST and are effective at up-regulating AR mRNA and protein expression that appears to correspond to subsequent increases in myofibrillar protein.