Increasing use of electromagnetic waves in living medium, industry, communication and medicine, strongly requires multi proposal evaluation of the effects of electromagnetic fields (emf), with different frequencies, on body tissues and organs. Contraversal reports on detrimental effects of emf on biological systems influenced us to study in this field. In order to evaluate the effects of uniform emf on biologic systems, we devised a calibrated emf unit (50-60 Hz, adjustable in 15 range which could produce up to 150 G emf). Six pair of pregnant rats rats were exposed on 120 G uniform electromagnetic field (50-60 Hz), for determining the effects of emf on development of embryonic cardiac muscle. They were daily exposed to emf during pregnancy and 2 month after birth for hours. Then, prepared histologic specimens of rats heart were studied by light microscopy. Histological studies showed that in experimental group in comparison to control group the size of cells and their nuclei were decreased remarkably and intercellular blood vessels were less extensive.The newly designed emf unit, in this research, has tuneable emf, large space for placing rats, better ventilation with automatic temperature control, and humidity and pressure meters. Therefore, it provides a better and suitable tool for studying the effects of electromagnetic fields. Exposure to emf resulted in decreasing cell growth and condensation of nuclei.