We have isolated and characterized seven members of the az,-globulin gene family from a rat genomic library. The 5' upstream region (up to 1250 base pairs starting from the EcoRI site in exon 2) of three clones was sequenced. The major transcriptional start points were located 25 base pairs downstream from the 'TATA' box. A very high degree of homology was observed over the entire studied region. Two of the examined genes displayed structural features which suggest that their expression may be impeded. A high degree of homology was observed between the promotor regions of a2,-globulin and those of the major urinary protein (MUP) multigene family of the mouse. A remarkable feature is the variable length of an A-rich region between the putative 'CAAT' and the 'TATA' consensus sequences. The size of this region differs markedly between MUP and a2,-globulin and between different members of the a2,-globulin gene family. Comparison of the a2,-globulin promotor with the corresponding region of other androgen-dependent genes (the C1, Cz and C3 subunits of prostatic steroid binding protein) reveals the presence of an A-rich region of homology located approximately 378 base pairs upstream from the cap site in the a,,-globulin genes. This region compares well with a sequence of putative enhancer function previously demonstrated in the a-fetoprotein promotor and in the immunoglobulin heavy chain promoter. a2,-Globulin is a protein of unknown function abundant in the urine of adult male rats but barely detectable in that of females. The majority of this protein is synthesized in the liver, secreted into the blood and, because of its small size (Mr = 18 700), excreted into the urine [l -31. The hepatic production of az,-globulin is under multihormonal control. Androgens are the most important physiological regulators but glucocorticoids, insulin, thyroxine and growth hormone are also required for normal synthesis [4-71. Oestrogens on the contrary [8, 91 and a factor secreted by ectopically transplanted pituitary glands [lo] suppress ~x~,-globulin. All these hormones control a2,-globulin synthesis mainly by regulating the hepatic level of a2,-globulin mRNA [ll-141. a2,-