The nucleus rotundus (ROT) is a major relay station in the tectofugal pathway of the avian visual system. In this study, some quantitative features of ROT in developing chicks were analysed using new stereological methods. Total neuron number (N) and mean volume (V) of ROT were estimated by the optical fractionator method and by the Cavalieri principle, respectively. Neuronal density of neurons in ROT was calculated from these data. The eyes of the chick embryo are not normally stimulated by light until days E19/20. Therefore in this study, chicks at three developmental stages were investigated: on the 17th embryonic day (E17), that is before light stimulation of the visual system, at the time of hatch (0-day, stimulated by light) and 10 days after hatch (10-day). The results showed that N was reduced by 27% between E17 and 0-day, and 7.8% between 0- and 10-day while neuronal density was reduced by 15% and 32% over the same periods. It is concluded that the reduction of neuronal density during the pre-hatch period may be due to neuron loss, whereas the post-hatch decrease of neuronal density may be the result of an increase in ROT total volume. Cell loss was more prominent in the pre-hatch than in the post-hatch period. Estimates of neuronal density in the developing ROT are not useful indicators of developmental status, since they do not relate to total neuron number.