The β₂-agonist clenbuterol [4-amino-α(t-butyl-amino)methyl-3,5-dichlorobenzyl alcohol] is used as a non-steroidal anabolic drug for sports doping. The effects of clenbuterol on the transcriptional process and mRNA stability of β-adrenoceptor (β-AR) in skeletal and cardiac muscles are still unknown. Therefore, we investigated the effects of clenbuterol on β₁- and β₂-AR mRNA expressions of fast-twitch fiber–rich extensor digitorum longus (EDL), slow-twitch fiber–rich soleus (SOL), and left ventricle (LV) muscles by real-time RT-PCR. Adult male Sprague Dawley rats were divided into the clenbuterol-administered group and control group. The administration (dose = 1.0 mg/kg body weight/day, s.c.) of clenbuterol was maintained for 10 days. The administration of clenbuterol significantly increased the weight, RNA concentration, and total RNA content of EDL muscle. No effects of clenbuterol on those of SOL and LV muscles, however, were observed. The administration of clenbuterol significantly decreased β₁-AR mRNA expression of LV muscle. Furthermore, the administration of clenbuterol significantly decreased β₂-AR mRNA expression of EDL and LV muscles. No effect of clenbuterol on β₂-AR mRNA expression of SOL muscle, however, was observed. These results suggest that the effects of clenbuterol on β₁- and β₂-AR mRNA expressions and muscle hypertrophy depend on muscle fiber types.