A transient interference experiment has been performed using the radiation from a normally spiking, pulsed ruby‐laser oscillator. In the experiment the laser output at timetis made to interfere with emission corresponding to an earlier time,t‐&tgr;, by means of an optical delay inserted into a two‐pinhole Young's interferometer. A light‐pipe photomultiplier oscilloscope system permits photographic recording of the interference fringes during a single spike of the laser emission, with a time resolution of less than 15 nsec. From the data obtained both the fringe visibility and the relative pattern phase of the interference pattern may be evaluated. Data are presented for both visibility and pattern phase for several delay times. The principal observation is that during a single laser spike the frequencyincreaseswith time. An analysis of the experiment and of possible causes for frequency variation of the laser with time results in the hypothesis that there is lattice strain in the ruby associated with the optical pumping to the excited state from which laser emission occurs. Linearity is assumed between strain and the number of excited ions. This permits a calculation to be made of the frequency change resulting from the strain and very reasonable agreement is obtained upon comparison with experiment. The visibility data are discussed but quantitative conclusions cannot be drawn.