The purpose of the experiment was to assess the effects of fluoride (F-) on the remodeling process of cortical bone. Sixteen pigs, eight experimental animals receiving a supplement of 2 mg F-/kg b.w. and eight controls, were studied in individual sites from age 8 to 14 months. At slaughter samples of cortical bone were obtained from the right femur and embedded undecalcified. A new stereologic model based on fluorochrome tissue time markers and isotropic uniform random histologic sections was implied in order to obtain information in three-dimensional terms about dynamic aspects of remodeling. The rate of remodeling was increased in cortical bone from pigs receiving F- due to an increased activation of new remodeling. A doubling of the length density of resorptive and formative osteons was observed, although the change was statistically significant for the formative osteons only. An 11% decrease in depth of resorption and an 8% decrease in thickness of new bone formed led to a small decrease in the radius of Haversian canals in the fluorotic bone. The porosity of cortical bone was slightly but significantly increased. At formative sites the osteoid thickness and the mineralization rate were not significantly changed by F-. It was concluded that the observed changes cannot be explained by F- induced changes in a single cell. Fluoride appears to affect all cells involved in remodeling by direct or indirect mechanisms.