Inotropic responses to 5-HT₄ receptor activation fade with time in porcine atrium. This has been attributed to the action of phosphodiesterases (PDEs), which hydrolyze the second messenger cAMP in cells. We assessed which PDE subtypes are responsible for this fade by testing selective PDE inhibitors alone and in combination on the inotropic responses elicited by 5-HT and by the 5-HT₄ receptor agonists prucalopride and RS67333. Porcine left atrial pectinate muscles were obtained from young male pigs (Seghers, breed line 12, 10-12 weeks, 15-25 kg), attached to tissue holders equipped with electrodes and mounted into tissue baths filled with aerated Krebs-Henseleit solution preheated to 37°C. Under a resting load of 2 g muscles were continually stimulated with square-wave pulses (0.5 Hz, 5 ms duration, 2-4 V). All inotropic responses were normalized to the response elicited by the b-receptor agonist isoprenaline added at the end of the experiment. The fade of the inotropic response to 5-HT₄ receptor activation could not be prevented by any selective PDE inhibitor alone (PDE2: EHNA; PDE3: Cilostamide; PDE4: Rolipram). The combination of cilostamide and rolipram increased the amplitude and completely prevented the fade of the response to 5-HT and prucalopride, similar to the unselective PDE inhibitor IBMX. Inotropic responses to RS67333 were only observed in the presence of IBMX or cilostamide plus rolipram. The combination of rolipram plus EHNA, but not cilostamide plus EHNA increased the amplitude of the inotropic response to 5-HT but only partially prevented the fade. Our results illustrate that the fade of the response to 5-HT₄ receptor stimulation is mediated by PDE3 and PDE4 subtypes in porcine left atrium.