We isolated four variants of ER alpha mRNA, two each from the brain and gonad of the Xenopus embryo (brain, bER alpha 1 and bER alpha 2; gonad, gER alpha 1 and gER alpha 2). The N-terminal-domain of ER alpha differed between the brain and gonad. While the C-terminal region downstream of the DNA-binding domain of ER alpha differed between ER alpha 1 and ER alpha 2 in both the brain and gonad, each of ER alpha 1 and ER alpha 2 contained the same sequence sets (bER alpha 1 and gER alpha 1; bER alpha 2 and gER alpha 2) between the brain and gonad. Both bER alpha 1 and gER alpha 1 contained the same full-length, C-terminal estrogen receptor, whereas the C terminus of bER alpha 2 and gER alpha 2 lacked half of the DNA-binding, ligand-binding, and transcriptional activation domain. Although gER alpha was mainly expressed in the gonad, both bER alpha and gER alpha were expressed in the brain of the Xenopus embryo at stage 50. These isoforms might be transcribed by alterative splicing of a single gene. Two gonad ER beta s differing in the length of the N-terminal sequence were isolated from the Xenopus gonad. While ER beta was expressed in the gonad, it was not expressed in the brain of the Xenopus embryo at stage 50.