Estrogens play an important role in tissue metabolism through specific regulation of several intracellular pathways. We studied ERalpha regulation in muscle and adipose tissue from pregnant and estradiol treated rats. In both groups, we identified three different ERalpha inmunoreactive proteins (80, 67 and 46 kDa) using total protein extracts. Because it has been showed that estrogens are able to promote rapid effects in several cellular models, we looked for three ERalpha-related proteins at plasma membrane. In skeletal muscle of both groups, we positively identified the three ERalpha-related isoforms in plasma membrane, but in adipose tissue from pregnant we were not able to identify ERalpha67, and in estradiol treated animals ERalpha80 was absent. Taking together, our results showed a tissue-specific regulation of whole-cell ERalpha-related proteins and ERalpha located at plasma membrane, which should be involved in non-genomic actions of 17beta-estradiol. The role of the three ERalpha inmunoreactive proteins is unknown, however, seems probably related to rapid activation of signalling pathways.