The complete mRNA sequence of the chicken progesterone receptor (cPR) has been determined. Expression of the cloned cDNA both in vivo and in vitro produces a protein that has the same apparent mol. wt on SDS--polyacrylamide gels as the 'natural' cPR form B (109 kd) as determined by immunoblotting and photoaffinity labelling. When expressed in HeLa or in Cos-1 cells the 'cloned' cPR displays hormone binding characteristics indistinguishable from the 'natural' receptor and, in the presence of progestins, exhibits 'tight nuclear binding'. A protein corresponding in size to the cPR form A (79 kd) could be detected by expressing in vivo and in vitro an N-terminally truncated cPR starting at methionine 128. A protein of the same apparent mol. wt results from internal initiation during in vitro translation. In contrast, such a protein was barely detectable after in vivo expression of the cPR cDNA in Cos-1 cells. These results suggest that form A is generated by an oviduct cell specific process involving either internal initiation of translation and/or proteolysis in the vicinity of methionine-128. The cPR contains two highly conserved regions C and E, a characteristic of the steroid/thyroid hormone receptor supergene family. By expression of a series of cPR deletion mutants, region E could be defined as the hormone binding domain whereas region C is indispensable for the tight nuclear association of the progestin-receptor complex. In the presence of progestins, the cloned cPR efficiently trans-activates transcription from the long terminal repeat region (LTR) of the mouse mammary tumor virus (MMTV). Deletion of the entire N-terminal region A/B or of the hormone binding domain E results in a 100-fold reduction of transcriptional activation. No stimulation of transcription can be detected when the C-terminal deletion extends into region C, indicating that this region is involved in the recognition of the hormone responsive element (HRE) of the MMTV LTR.