The applicability of flow cytometry (FCM) to analyse cell-cycle distribution and mitotic cells in Hydra oligactis and Hydra vulgaris is demonstrated. The freshwater polyps H. vulgaris and H. oligactis are well-accepted animal models for studying cell proliferation, regeneration and differentiation. Disintegrated animals were labelled for FCM analysis according to the method of Nuesse et al. [(1990) Flow cytometric analysis of G(1) and G(2)/M-phase subpopulations in mammalian cell nuclei using side scatter and DNA content measurements. Cytometry 11, 813]. Proliferation and regeneration experiments, in the absence or presence of the oligopeptide head activator, were quantified. Cell-cycle analysis of different parts of the animals shows low proliferation in the head region and high proliferation in the gastric and foot regions. Cell-cycle analysis of different parts of Hydra, comparison of H. oligactis and H. vulgaris, as well as pharmacological treatment, yielded results that are in agreement with prior microscopic analysis. Our results demonstrate that FCM is an appropriate technique for quantifying proliferation in this animal model. It can be used for basic research on development, regeneration and differentiation as well as for innovative drug investigation and toxicology studies.