Today's stringent requirements for new drugs make it necessary to prove their safety by the use of ever more refined techniques, in particular methods to quantify morphologic findings to determine accurately, for example, the highest non-toxic dose in animal studies or to correlate morphological effects with other parameters. Increased cell proliferation due to subtle cytotoxicity can lead to epigenetic tumorigenicity, which can be recognized early by cytokinetic investigations using bromodeoxyuridine (BrdU) incorporation and/or proliferating cell nuclear antigen (PCNA) expression, or by malignancy-associated nuclear texture changes. Morphometric criteria are also used to increase diagnostic accuracy in experimental animal pathology. The various applications of morphometry in toxicopathology are reviewed, and examples illustrating these applications are presented, most of them unpublished.