The pattern of distribution of SP40,40-like immunoreactive structures has been studied in the rat brain using a well-characterized polyclonal antibody raised against the SP40,40 protein. Protein SP40,40 is the human counterpart of the rat sulphated glycoprotein 2, whose mRNA shows widespread expression in the developing and mature brain. In young adult rats few immunoreactive structures were observed. Some immunoreactive neurons were found in the cingulate cortex, the arcuate and perifornical hypothalamic nuclei, as well as glial labelling in the hypothalamus. A striking increase in the number of immunoreactive cells was observed as a function of age. In 20–22-month-old rats, numerous immunoreactive cells were observed in the cingulate cortex, several thalamic and hypothalamic nuclei, the red nucleus, olivary nuclei, superior colliculus, and many cranial nerve nuclei. Whereas the immunoreactivity was restricted to a diffuse labelling of the cell bodies and processes in young rats, other forms of labelling were observed in aged rats: punctate cytoplasmic labelling and intensely stained granules with no visible cell membrane. A further increase in the density of the immunoreactive material was observed in 30–31-month-old rats. Double labelling experiments demonstrated that the SP40,40 immunoreactivity was almost exclusively located in neurons and not in glial cells (with the exception noted above). The distribution of SP40,40 immunoreactivity in aged rats did not coincide with the distribution of the microtubule-associated tau protein, OX42 or lipofuscin.