N,N-Dimethyl-6-dodecanoyl-2-naphthylamine (Laurdan) is used as an environmentally sensitive probe for the labeling of plasma membranes. The lauroyl-sidechain integrates into the hydrophobic core of biological membranes, the polar fluorescent moiety is at level of the glycerol-backbone. Dipolar relaxation processes of the naphtylamine-moiety with neighboring polar molecules lead to wavelength-shifts of the emission-spectra. The physical state of the membrane and effects of temperature, cholesterol or ethanol on various membrane-systems have been monitored.