Development of the nasal capsule cartilages was studied in seven Geoffroy's tamarins (Saguinus geoffroyi), including one fetus, five neonates and one infant. Four additional postnatal specimens of the genus were studied (one 5-month-old and three adults) to determine the magnitude of postnatal expansion of the paranasal sinuses. Alcian blue histochemistry and osteopontin immunohistochemistry were employed in selected subadult specimens to characterize cartilage matrix. The fetal S. geoffroyi possesses a continuous nasal capsule, including a zona anularis; the primordial maxillary sinuses are surrounded by cartilage. Secondary pneumatization is in progress in all older specimens, which have sinuses that are more than twofold larger compared to that of the fetus. Results indicate that extensive ossification of the middle part of the nasal capsule (pars intermedia) is occurring in the perinatal timeframe, forming portions of the ethmoid bone. Anteriorly, the nasal capsule comprises isolated fragments in perinatal specimens, which are fewer and smaller in the infant and in a 5-month-old S. midas, and nearby multinucleate cells suggest that osteoclasts break apart these initially continuous elements. Fragments of the pars intermedia and the tectum nasi are found transiently between mucosa and the sites of secondary pneumatization. The maxillary sinus mucosa is highly vascular in most perinatal specimens. Histochemical and immunohistochemical findings show that cartilage of endochondral bones and non-ossifying parts are distinct in the perinatal time period. These results indicate that breakdown of the capsular cartilage precedes secondary pneumatization as previously suggested. There are portions of the cartilage of the recessus maxillaris and tectum nasi that transiently block mucosa from interfacing directly with bone. Vascularization may play a role in the breakdown of cartilages as well as the onset of secondary pneumatization. Since cartilage has the capacity to produce substances that trigger angiogenesis and bone resorption, further detailed characterization of the cartilage bordering sites of secondary pneumatization is merited. Anat Rec, 291:1397–1413, 2008. ©2008 Wiley-Liss, Inc.