Caveolae are abundant in adipocytes and are involved in the regulation of lipid accumulation, which is the main volume determinant of these cells. We have developed and applied a confocal microscopic technique for measuring individual cellular expression of the caveolar proteins cavin-1 and caveolin-1 along with the size of individual adipocytes. The technique was applied on collagenase isolated adipocytes from ad libitum fed Sprague-Dawley rats of different age (4-26 wk) and weight (103-629 g). We found that cellular expression of caveolar proteins was variable (SD of log expression in the range from 0.25 to 0.65). Regression analysis of protein expression on adipocyte size revealed that the expression of the caveolar proteins cavin-1 and caveolin-1 on adipocytes from individual rats was tightly related to adipocyte cell surface area (mean coefficient of regression was 0.83 for cavin and 0.77 for caveolin), indicating that caveolar density was the same in membranes from all cells within a biopsy. This intrinsic relation remained unchanged with animal age, but adipocytes from animals with increasing age showed a decrease in mean expression of caveolar proteins per unit cell surface. The different relation between adipocyte size and cellular expression levels of caveolar proteins within and between individuals of different age shows that caveolar density is an age-sensitive characteristic of adipocytes.