A search for X-ray sources exhibiting pulsating characteristics similar to the ones recently discovered in Cyg X-1 by Oda et at., and confirmed by Holt et at., has revealed the existence of periodic pulsations in the X-ray emission from Cen X-3. The pulsations are of large amplitude, comprising at least 70 percent of the total flux from the source. The period is a proximately 5 5. Due to the amplitude of the effect and the long period, we are able to establish that t e period and phase of the pulsations are maintained for several hours. In addition, two very remarkable events are observed to occur during a 1-day interval. The first is a change in the intensity of the source by a factor of 10 occurring in approximately 1 hour. The second is a decrease and then an increase in the period of the pulsations, corresponding to about 0.02 and 0.04 percent, each change of period occurring in a time of about 1 hour. The magnitude of the changes, and the short time interval in which they take place, are quite different from any observed in radio pulsars. In addition, we see evidence for changes of the period by about 1 percent occurring over a time span of 3 months. Present data are inconclusive about the presence of faster pulsations from the source. The X-ray spectrum shows evidence of substantial absorption below 3.8 keV.