This article develops a varying-parameter advertising model which specifies advertising parameters as a function of variables representing advertising strategies and market environments to explain the varying nature of the advertising responses. Unlike prior models, this model allows researchers to examine the sources of change in advertising effectiveness over time. The model is applied to the New York City fluid milk market for the period from January 1986 through June 1995. Results indicate that advertising strategies and market environments play important roles in determining advertising effectiveness. Particularly, demographic factors were more important than economic factors. The results also suggest that when a market is in an unfavorable or unsaturated condition, advertising generally becomes more important and effective.