Noting the agenda-setting hypothesis that the public will adopt the media's agenda of issue priorities after the passage of some time, a study examined the effect of time on the public agenda. The study measured responses of 880 subjects in three survey waves. Evaluations of the importance of seven environmental issues were correlated with week-long intervals of media coverage of those issues for up to 33 weeks preceding each survey. The results indicated that the media had their optimal impact on the public's perceived importance of environmental issues after eight to ten weeks of media coverage. The findings also showed that the correlations between the media agenda and the public agenda tended to increase, peak and level, and then decline with the passage of time. Possible explanations for this effect are discussed. Tables and references are appended. (Author/HTH)