Heart tissue contains cardiac progenitor cells (CPC). The reparative potential of these cells is extensively discussed. Paracrine mechanisms become more and more presumably but remained still very speculative. In order to identify these paracrine mechanisms, CPC of the human heart were isolated (Messina et al.). Therefore human donor tissue was classified into two groups (juvenile: 2 month to 9 years (CPCjH); adult: 20 to 50 years (CPCaH)). Conditioned media of these ex-vivo generated CPC were used to study contractile function of adult isolated cardiomyocytes of rats after 24h of stimulation. As control the respective pure media were used. Additionally hypertrophy-associated, apoptosis-associated and Ca²⁺-regulating proteins of the incubated myocytes were examined by western blotting. 507 different cytokines were examined by cytokine array. In the present study the effects of a total of 11 different CPC (7 juvenile and 4 adult) preparations were characterised, investigated and compared. Besides the immune-histochemical characterisation of the generated CPC, Ca²⁺ transients and contractions were additionally measured. Spontaneous Ca²⁺-transients and contractile activity increased by the grade of differentiation. Stimulation with isoprenaline in a concentration of 100nM increased and addition of a RyR2-antagonist (10mM) completely inhibited both. Conditioned media of these CPC cultures increased contractile function (Dl/l) of isolated myocytes by 25.2% 9.7% (CPCjH) and by 28.7% 7.5% (CPCaH) versus the respective pure media. The Serca/NCX ratio increased in the stimulated myocytes whereas other parameters like TGFbeta, ANP, BNP, bax and bcl-2 expression remained stable. 42 of the examined cytokines were intensively released. From these 42 determined cytokines VEGF, GDF-15, and IL-8 increased contractile function (Dl/l) significantly, whereas TIMP-1, GRO, APRIL and TACI exerted no and IL-1beta even a negative influence on contractile function. This study verified the paracrine mechanisms of CPC independent of the donor's age. If some of the identified released factors or the multiplicity of them are responsible for the determined effects need to be answered in further studies.