Background: Zebrafish are increasingly used to study the influences of gene mutation and manipulation on cardiac development, structure and function. In this study, a video edge detection system was used to characterise, continuously, cardiac ventricle function in 2-5 days old zebrafish embryos embedded in 0.6% agar and examined under light microscopy at room temperature (22 C). Using video edge detection software (IonOptix Inc), the motion of a small region of the cardiac ventricle wall was converted to a continuous chart trace allowing analysis of wall motion amplitude (WMA) and myocardial wall velocity during systole (MWVs) and diastole (MWVd). Results: Cardiac wall motion characteristics changed progressively from day 2 to 5 (WMA, 2-days, 17.6 +/- 4.4 mu m vs 5-days, 24.6 +/- 4.7 mu m, p <0.01). MWVd was more rapid than MWVs at all developmental time points. Embryonic hearts were also assessed after increasing concentrations of norepenephrine (NE) and the anaesthetic agent MS222 (tricaine) were added to the bathing water. In response to NE, WMA increased significantly more in 4 day embryos compared with 2 day embryos (change in WMA, 13.6 +/- 8.2 mu m vs 4.0 +/- 8.8 mu m, p = 0.01, respectively) while the decrease in WMA in response to MS222 was similar in both 2 and 4-day embryos. Heart rate, MWVs and MWVd were significantly higher at 28 degrees C compared with 22 degrees C. No differences in cardiac function were observed between AB and Golden strains. Conclusion: Video edge detection appears sufficiently sensitive to detect subtle changes in diastolic and systolic cardiac function during development and changes resulting from pharmacological and environmental interventions. Such measurements could be valuable in assessment of altered cardiac function after genetic manipulation.