This study was aimed at determining whether the chick cardiomyocyte micromass (MM) system could be employed to predict the teratogenicity/embryotoxicity of exogenous chemicals. Two documented teratogens/embryotoxins, sodium valproate (the sodium salt of valproic acid; VPA) and all -trans retinoic acid (tRA), were used in the initial phase of the study. White Leghorn 5-day-old embryo hearts were dissociated to produce a cardiomyocyte suspension in Dulbecco's Modified Eagle's Medium. Cultures were incubated at 37°C in 5% CO2 in air, and observations were made every 24 hours over 5 days, for the detection of beating. Culture viability was assessed by using the resazurin reduction assay for determining culture activity and the kenacid blue assay for determining cell number. It was found that tRA significantly reduced cell activity and beating, whilst not affecting total cell number. VPA up to 500μM induced no cytotoxicity in the MM cardiomyocyte cultures, whilst all the VPA concentrations tested reduced beating. The results demonstrate the potential of the chick cardiomyocyte MM culture assay to identify teratogens/embryotoxins that alter functionality, which may result in a teratogenic outcome, whilst not causing cytotoxicity (direct embryotoxicity). This could form part of a screen for developmental toxicity related to cardiac function, whilst limb cultures and brain cultures based on the same system could be relevant to teratogenic effects on those tissues.