Detailed simultaneous visible and ultraviolet spectroscopy of Zeta Puppis was carried out in order to study the time scale of variations in its stellar wind and to assess the relationship between fluctuations in lines of ions formed at different heights. The observed variations were most striking in the He II 4686 A profile, but were also seen in H alpha and in the resonance doublets of Si IV and O VI. Fluctuations in the visible features occurred in times of about one hour, while the ultraviolet profiles showed changes which took several hours to develop. No evidence of periodicity was seen, but the time coverage is not sufficiently complete to preclude it. Specific changes, particularly in H alpha and 4686 A, may have been caused by the passage of density enhancements from the stellar surface outward through the wind. The profile variations, when compared with recent model calculations, imply changes in the mass-loss rate by no more than a factor of two in times of a few hours.