A nanosecond fluorimeter suitable for measuring the emission kinetics of chromophores which have excited state lifetimes longer than 1 nsec is described in this paper. The acquisition and processing of the nanosecond data were accomplished in entirety by computers. The pulsed light source, an oxygen spark‐gap lamp operated in a relaxation mode, is simple to construct and use. The lamp exhibits high intensity over a broad spectral range, as well as a short pulse duration. In the wavelength interval from 230 to about 470 mμ, there were 6×1011 photons per pulse at a repetition rate of 2.1 kHz. The spectrum of the lamp was approximately flat between 200 and 600 mμ. The rise and fall times of the light pulse, as measured with a planar photodiode, were 0.7 and 1.1 nsec. The accompanying current pulse had rise and fall times shorter than 0.43 nsec. A 1P21 photomultiplier tube was used as the detector in the fluorimeter. The output of the 1P21 was connected to a sampling oscilloscope that was triggered by a synchronous current pulse from the light source. The analog output of the sampling oscilloscope was digitized, stored, and averaged on a LINC computer, and then transferred to magnetic tape for processing on an IBM 7090. The nanosecond data were analyzed in terms of a convolution integral which took into account the shape of the exciting light pulse as seen by the detection system. The use of the apparatus is illustrated by two examples: 1, 4‐bis‐2‐ (4‐methyl‐5‐phenyloxazolyl) benzene, which has a lifetime of 1.5 nsec; and 1‐anilino‐8‐naphthalene sulfonate bound to serum albumin, which has a lifetime of 17.1 nsec.