The green-fluorescent protein (GFP) from Aequorea victoria has been shown to be a convenient and flexible reporter molecule within a variety of eukaryotic systems, including higher plants. It is particularly suited for applications in vivo, since the mechanism of fluorophore formation involves an intramolecular autoxidation and does not require exogenous co-factors. Unlike standard histochemical procedures of fixation and staining required for analysis of the cellular or tissue-specific expression of other popular reporter molecules, such as the beta-glucuronidase (GUS) marker, analysis of GFP can be done in living cells with no specific pretreatments. This implies that GFP might also be particularly suited for studies of intracellular protein targeting. In this paper, the use of GUS is compared with that of GFP for the analysis of nuclear targeting in tobacco. A novel oligopeptide motif from a tobacco protein is described which confers nuclear localization of GUS. The use of this oligopeptide and two from potyviral proteins to target GFP to the nucleus is examined. An essential modification of GFP is described, which specifically increases its molecular weight to eliminate its passive penetration into the nucleus. Three examples of the targeting of these enlarged GFP molecules to the nucleus are illustrated. GFP, in combination with confocal microscopy, offers significant advantages over traditional methods of studying nuclear targeting.