Scientific Journal Paper I-933619 of the Alabama Agricultural Experiment Station.This paper adduces and tests the hypothesis that generic advertising responses are dynamic, i.e., are subject to change over time due to changes in target audiences, managerial expertise, copy quality or other time-related factors. Specifically, the authors consider the structural heterogeneity hypothesis from the perspective of three alternative econometric models that permit random ana' systematic timevarying response: the Prescott-Cooley model, the return-to-normality model, and the stochastic-trend model. A distinguishing characteristic of the models is the presence or absence of heteroscedasticity. Based on pretests, which failed to detect heteroscedasticity, a modified version of the stochastic-trend model is selected for hypothesis testing. Results based on data of the first 15 years of the Ontario fluid milk campaign suggest advertising responses are dynamic. Estimated advertising elasticities decline more or less monotonically over this sample period, from a high of 0.020-o. 031 in the initial 1973-74 theme period, to a low of 0.0004-o. 009 in the final 1986-87 theme period. The apparent declining eflectiveness of the Ontario fluid milk campaign is consistent with wearout theory, and suggests that program managers may want to reassess marketing strategies to identify possible ways to improve performance. Given the importance of advertising elasticities in normative decision models and the growing evidence of structural heterogeneity, models that permit parameters to change over time should provide an improved basis for program assessment and resource allocation. Les auteurs elaborent et testent 1 'hypothese selon laquelle les reactions d la publicite generique sont dynamiques, c 'est-a-dire qu 'elles peuvent evoluer dans le temps sous 1 'effet de changements affectant entre autres les auditoires cibles, 1 'expertise de gestion, la qualite'du message publicitaire. Plus precisement, ils testent 1 'hypothbe de 1 'hett?rogtWite' structurelle a partir de trois modeles e'conomhriques alternatifs autorisant une reponse aleatoire et systematique en fonction du temps: le modele Prescott-Cooley, le modele du retour a la normalite et le modele des tendances stochastiques. Une caracteristique distinctive de ces modes est la presence ou 1 'absence d 'htWrosc&lasticite. Des tests preliminaires n 'ayant pu deceler aucun signe d 'het&osc&'asticite', une version modif%e du modele des tena'ances stochastiques etait retenue. Les resultats bases sur les don&es des 15 premieres annees de campagnes de promotion pour le lait de consommation en Ontario laissent voir que les reponses a la publicite sont dynamiques. L.es elasticites calculees de 1 'efi?cacite' de la publicite diminuent selon un mode plus ou moins monotone d 'une periode d 'echantillonnage a 1 'autre, soit d 'un maximum de 0,020-0,031 darts la phase thematique initiale de 1973-I 974, h un minimum de O,ooO4-0,009 a'ans la phase thematique finale de 1986-I 987. La baisse apparente d 'ef...