A study was carried out on the secretion organelles of thyroid follicular cells during postnatal (PN) development in normal conditions and after irradiation at 46.8 J/cm 2 with an infrared (IR) laser (904 nm). This was done using 11-, 21-and 35-day-old Wistar rats. The changes in the organelles were determined using stereological methods (volume and surface density) as well as electron microscopy. During normal PN development, the parameters corresponding to the di#erent organelles of follicular cells show very high values in the 11-day-old animals, decrease in the 21-day-old animals, especially in mitochondria, then increase again in the 35-day-old animals. The results obtained show that follicular cells reach maturity early during PN development and present fluctuations in their activity during the period studied. Radiation with IR laser (46.8 J/cm 2 ) does not seem to alter the PN development of the cells, but does induce changes in the secretion organelles, especially at day 35, among which the increase of colloid droplets and lysosomes, dilation of rough endoplasmic reticulum, and reduction of the Golgi apparatus stand out. We believe these alterations could modify thyroid hormone synthesis and secretion processes.