By use of in situ hybridization experiments, the transcripts of several different mouse homeo-genes ( Hox-1.2 , -1.3 , -1.4 , -1.5 , -3.1 and -6.1 ) have been localized in l2½-day mouse embryos. In a comparison of these genes on adjacent or nearby embryo sections, it is found that their transcripts occupy domains which are usually different, although overlapping, along the anteroposterior axis of the body. The domains are not limited to single segments (assumed to be represented by single prevertebrae) but they encompass regions of adjacent segments. In addition to the prevertehral column, the transcript domains extend into the central nervous system and at least some of the organs (pharynx, thyroid, trachea, lung, stomach and kidney). Within the prevertebral column, a striking feature of most of the domains is that the abundance of transcripts rises (anteriorly) and falls (posteriorly) over a distance of several adjacent prevertebrae. For Hox-1.4 and Hox-1.3 the rise is over prevertebrae within the cervical region. For Hox-6.1 , Hox-1·2 and Hox-3.1 , the rise is over prevertebrae within the thoracic region. For each of the genes examined, transcripts in the central nervous system extend to a more anterior position in the body than transcripts in the prevertebral column. The myelencephalon of the hindbrain contains at least three different anterior boundaries for homeo-gene transcript domains. The positions of these are defined by Hox-1.5 (most anterior), Hox-1.4 and Hox-1.3 . Anterior boundaries for Hox-6.1 and Hox-1.2 are apparently located at the Hox-1.3 position. Homeo-gene transcript domains extend into several structures known to be derived, at least in part, from the neural crest. These include the ventral pharynx, thyroid, aortic trunk and, probably, the sympathetic nerve chain and thymus. For several genes of the Hox-1 clusterr we note a correspondence between the serial arrangement of genes on the chromosome and the arrangement of their transcript domains in the developing embryo. We also note some striking similarities between the transcript domains of different homeo-genes that share the same subfamily (Duboule et al . 1988). These observations, and others, offer possible clues about the molecular mechanisms that might underlie the formation and maintenance of homeo-gene transcript domains.