Fetal cardiac growth in mammalian models occurs primarily by cell proliferation(hyperplasia). However, most cardiomyocytes lose the ability to proliferateclose to term and heart growth continues by increasing cell size(hypertrophy). In mammals, the thyroid hormone triiodothyronine (T3) is animportant driver of this process. Chicken cardiomyocytes, however, keep theirproliferating ability long after hatching but little information is available onthe mechanisms controlling cell growth and myocyte maturation in thechicken heart. Our aim was to study the role of T3 on proliferation and differentiationof embryonic chicken cardiomyocytes (ECCM), enzymaticallyisolated from 19-day-old embryos and to compare the effects to those of insulin-like growth factor-1 (IGF-1) and phenylephrine (PE). Hyperplasia wasmeasured using a proliferation assay (MTS) and hypertrophy/multinucleationwas analyzed morphologically by phalloidin staining of F-actin and nuclearstaining with DAPI. We show that IGF-1 induces a significant increase inECCM proliferation (30%) which is absent with T3 and PE. PE induced bothhypertrophy (61%) and multinucleation (41%) but IGF-1 or T3 did not. Inconclusion, we show that T3 does not induce maturation or proliferation ofcardiomyocytes, while IGF-1 induces cardiomyocyte proliferation and PEinduces maturation of cardiomyocytes.