Neurons from the embryonic brain of Xenopus were transfected in vivo with a vector expressing luciferase cDNA using a simple lipofection procedure. Luciferase activity was monitored quantitatively, and the protein was immunolocalized in whole-mount embryonic brains. Luciferase-expressing neurons were often intensely labeled, displaying a Golgi-like filling of their dendrites, axons, and growth cones. Luciferase expression could be targeted to the retina by simply removing the skin epidermis covering the area and exposing the whole embryo to the DNA-lipofectin mixture. Luciferase activity in transfected embryos rose to peak values during the first 48 hr posttransfection and was still detectable 28 days later. Cotransfection experiments in which embryonic nervous tissue was exposed simultaneously to two different genes, luciferase and chloramphenicol acetyl-transferase, showed that transfected cells coexpressed the two genes at an extremely high frequency (85%-100%). This offers the possibility of targeting functionally significant genes along with benign reporter genes in the developing CNS.