1. An apparatus for 3-dimensional imaging of fluorophore-labeled molecules in a sample, comprising in combination: a pulsed activation laser for generating photons having wavelengths twice the wavelength of a chosen two-photon absorption of the fluorophores, said activation laser having sufficient power to generate two-photon absorption in the fluorophores; a pulsed light source for generating photons effective for producing fluorescence in the fluorophores, when photons from said activation laser are not being directed into said sample; a focusing lens for focusing photons generated by said activation laser and said fluorescence-generating laser onto a chosen area of said sample, and for collecting fluorescence radiation therefrom; means for directing the photons from said activation laser onto the surface of said sample in the x- and y-directions perpendicular to the direction of the photons; means for moving said sample in the z-direction, perpendicular to the plane formed by the x- and y-directions; a camera for measuring the fluorescence from the chosen area of said sample in each z-location; and time-gating means disposed between said focusing lens and said camera for letting fluorescence light reach said camera a chosen time after the pulse of photons from said fluorescence-generating laser impinges on said sample.