We have identified and characterized two nuclear progestin receptor isoforms (nPR-B and nPR-A) in zebrafish. The longer nPR isoform, nPR-B, contained 618 amino acid residues and composed of 5 typical domains of the steroid receptors. The nPR-A contained the same amino acid residues and 5 typical domains of the receptor with the exception of 110 amino acid residues missing in the variable region (A/B domain) of the receptor at the N-terminus. The identity of nPR was confirmed by: 1) high amino acid sequence homology with ligand binding domain of the zebrafish nPRs (68-83% aa identity) compared to known nPRs of other vertebrate classes; 2) nPR mediated progestin-specific and dosedependent activation of transcription; 3) abundant expression of nPRs in reproductive tissues including ovaries, testis and brain; 4) predominant expression in follicle cells, consistent with their role in ovulation.