In newly hatched brine shrimp (Artemia franciscana) cardiac organogenesis and function could only take place with the onset of segmentation. Consequently differences in age, body size and temperature (in the range 22-34 degrees C) affected the ontogeny of cardiac activity only indirectly, through their influence on developmental stage. Once present the frequency of cardiac activity (heart beat) increased with increasing body size and concomitant differentiation of cardiac tissue. At least initially, (dry body weight 11-16 micrograms) heart rate was independent of temperature and weight specific heart rate was independent of body size. When differentiation neared completion, and cardiac growth switched to elongation (dry body weight > 16 micrograms), there was a change in the pattern of cardiac function. There was now a decrease in heart beat with increasing body size and weight specific heart rate showed an inverse relationship with body weight. Heart rate also became temperature dependent, although it remained relatively insensitive. In conclusion it is suggested that the ontogeny of cardiac function in Artemia is constrained by an anamorphic (direct) pattern of development where segment formation occurs post hatch. Early cardiac functioning cannot be predicted on an allometric basis, as cardiovascular structure and function changes qualitatively during ontogeny.