The rice protein prepared from alkaline extraction (AE-RP) has high digestibility compared to that obtained from starch degradation (SD-RP) in in vitro digestion experiments, and alterations in the protein body (PB) structures were observed in AE-RP in the previous study. The improvement in the digestibility of AE-RP is probably a result of the structural change of PB. The present study was carried out to elucidate the superiority of AE-RP compared to SD-RP in bioavailability in growing rats. There were no major differences between AE-RP and SD-RP in polypeptide compositions according to SDS-PAGE and their amino acid compositions. The equivalent body weight gain and similar growth curves in both AE-RP and casein (control) groups were obtained during the feeding period of 28 d, and their values were significantly higher compared to the SD-RP group (p<0.05). The protein efficiency ratio (PER) of the SD-RP (1.73) group was significantly lower than those of the AE-RP (1.87) and casein (1.84) groups (p<0.05). The plasma lysine concentrations at the last stage of the feeding period in the AE-RP and SD-RP groups were approximate levels and were appreciably lower, compared to that of the casein group (p<0.001). Portal plasma amino acid concentrations were determined after single administration (4 g/kg) of two rice proteins in non-anaesthetized rats. All the amino acid concentrations in the 2 groups reached a maximum level at 30 min or 1 h and decreased to the pre-administration levels 6 h after the start of administration. The total amounts of three amino acids, leucine, valine and arginine, which appeared in the portal blood during the 6 h period after the start of administration of AE-RP, were higher than those of SD-RP (p<0.05). Furthermore, 13 kDa prolamin was detected with Western-blot analysis only in the feces of rats fed SD-RP. Consequently, these results indicate that the bioavailability of rice protein containing prolamin was improved by alkaline extraction.