Background: Members of the Peroxisome Proliferator Activated Receptor, PPAR, subfamily of nuclear receptors display complex opposing and overlapping functions and a wide range of pharmacological and molecular genetic tools have been used to dissect their specific functions. Non-agonist bound PPARd has been shown to repress PPAR Response Element, PPRE, signalling and several lines of evidence point to the importance of PPARd repressive actions in both cardiovascular and cancer biology. Methodology/Principal Findings: In this report we have employed transient transfections and luciferase reporter gene technology to study the repressing effects of PPARd and two derivatives thereof. We demonstrate for the first time that the classical dominant negative deletion of the Activation Function 2, AF2, domain of PPARd show enhanced repression of PPRE signalling in the presence of a PPARd agonist. We propose that the mechanism for the phenomenon is increased RXR heterodimerisation and DNA binding upon ligand binding concomitant with transcriptional co-repressor binding. We also demonstrated ligand-dependent dominant negative action of a DNA non-binding derivative of PPARd on PPAR?1 signalling. This activity was abolished upon over-expression of RXRa suggesting a role for PPAR/ cofactor competition in the absence of DNA binding. Conclusions/Significance: These findings are important in understanding the wide spectrum of molecular interactions in which PPARd and PPAR? have opposing biological roles and suggest novel paradigms for the design of different functional classes of nuclear receptor antagonist drugs.