Mitochondria (m) are responsible for the energy availability of cells, and their analysis is indicated for example, in studies related to metabolism and oxidative stress. The direct measurement of mitochondria (morphometry) is biased because of the section obliquity and position relative to the mitochondria length (non-equatorial cut). Therefore, stereology is an appropriate technique to evaluate mitochondria. However, before beginning the study, it is necessary to consider the premises to obtain random and uniform samples to be analyzed stereology. Mitochondria must have the chance to appear in all the possibilities of cut and orientation in the micrographs. The number of micrographs to be analyzed will depend on the distribution and occupation of mitochondria in the cell. After this is resolved, a proposal is the estimation of the following stereological data: volume density (Vv), surface density (Sv), and mean cross-sectional area (A). Overlapping a known test area at each micrograph, the density by area of mitochondria is estimated (N). Vv [m] can easily be estimated AT by point-counting (Vv = Pp/P ; Pp are the points hitting the structure, P are the number of points of the test system). Sv is estimated TT overlaying a test-line (L ) on the micrographs and counting the intersections of the lines (I) with the outer membrane (om), inner membrane T (im), and crests (c), thus, Sv [om], Sv [im], Sv [c] (Sv = 2I / L ). A [m] is obtained as the ratio: A = Vv / 2N . T AT

KEY WORDS: Mitochondria; Morphometry; Stereology; Microscopy; Cell biology.