A real time noninvasive method for evaluating regulation of gene expression in living mammals during development would facilitate our understanding of the many interwoven processes that direct mammalian ontogeny. Regulatory elements from viruses fused to reporter genes can serve as illuminating probes for ongoing cellular changes during developmental processes. We noninvasively evaluated a transgene consisting of the promoter from human immunodeficiency virus (HIV) and the coding sequence from the firefly luciferase gene, in groups of living transgenic neonatal mice. Bioluminescent light transmitted through the animals' tissues was externally monitored with an intensified CCD-camera, and followed over time. Differential light emission was observed throughout the first month of life; levels of luciferase activity in the developing eyes and extremities were elevated in comparison to basal levels of expression detected in other regions of the animals. Levels of bioluminescence in the eyes decreased to background at one week of age, and although variable, signals above background persisted in the extremities. At four weeks of age, luciferase expression was not apparent in any tissue; however, the promoter could be activated in the skin through topical treatment with dimethyl sulfoxide (DMSO), a known inducer of this viral promoter. Such activation of the promoter was apparent only after 17 d of age, possibly representing another developmental step. Regulation of expression at these transition points in murine development is likely mediated by interaction of cellular transcription factors with one or more of the six known binding sites in the promoter region. Probing developmental steps via noninvasive monitoring of gene expression will facilitate dissection of the tissue- and time-specific regulatory processes that direct mammalian development.