Morphometric parameters of mitochondria such as volumetric density, surface density of envelopes and cristae and numerical density were calculated for control untreated and triacetyloleandomycin (TAO)-treated rat hepatocytes using a point-counting technique. Moreover, morphometric parameters calculated experimentally and those computed on the basis of the least square interpolation and the agreement with the chi2 test, were compared. The equations modelling each main morphometric parameter of experimental motochondria were also computed. Morphometric study revealed that TAO produces a highly reproducible pattern of morphological alterations in the mitochondrial population of rat hepatocytes, and a good agreement between experimental and theoretical data was found. The response of hepatocyte mitochondria to TAO may be modelled by parabolic functions described by equations of the second degree. During the whole experimental period, the number of mitochondria decreases, but the specific volume of mitochondria increases. The area of the cristae surface per mitochondrion does not change substantially during the whole experimental period but since the quantity of internal mitochondrial membranes per hepatocyte is less in the later experimental period than in control material, it can be assumed that the oxidizing capacity per hepatocyte has diminished. The morphometric model based on TAO-treated hepatocyte mitochondria is different from those so far described in the literature for rat liver.