Abstract Enzyme activity and fine structure of thyroid follicular cells were investigated in domestic cats at ages: 2 and 8 months, and 1, 3, 5 and 8 years. The following major changes in the follicular cells with advancing age were observed: with histochemistry — an increase in acid phosphatase and beta-glucuronidase activities confined to numerous granules (vesicles); with electron microscopy — an increase in colloid vesicles, the appearance and increase in size and number of lipofuscin vesicles, and the increase in number of follicular cells containing numerous colloid or lipofuscin vesicles. It is our opinion that the numerous enzymatically active granules of the older groups, observed with histochemistry, are synonymous with the lipofuscin vesicles and many of the colloid vesicles observed with electron microscopy. The cellular changes involving increased enzymatic activity and lipofuscin accumulation with advancing age would correlate with decreased thyroid function reported in the literature. These observations, therefore, would suggest increased autophagic activity involving organelle or unneeded product degradation.