A cDNA coding for a human brain adenylyl cyclase was isolated and sequenced. The deduced partial 675 amino-acid sequence was compared with those of other known adenylyl and guanylyl cyclases. Comparison of this predicted amino-acid sequence with that of bovine brain (type I) and rat olfactory (type III) adenylyl cyclase indicated a significant homology with the carboxyl-terminal halves of both enzymes. The homology between the human adenylyl cyclase and the other two mammalian adenylyl cyclase also appears at the topographic level. Indeed, the human enzyme includes a extremely hydrophobic region containing six potential membrane-spanning segments followed by a large hydrophilic domain. At the beginning of the hydrophilic domain, there is a 250 amino-acid region which shows not only a striking homology with the bovine and rat adenylyl cyclase (86% of similarity and 57% of identity), but also a significant homology with non-mammalian adenylyl cyclase and guanylyl cyclases. We found that this 250 amino-acid domain contains a sequence of about 165 amino-acids which is highly conserved in most of the known nucleotide cyclases suggesting that it includes residues that are critical for the function of the enzymes.