The techniques of morphometric analysis were used to study the ultrastructural changes induced inPlectonema boryanum (Cyanophyceae) by heavy metal exposure. Four of the test metals (Mn, Zn, Hg, and Cd) produced no changes in overall cell dimensions. Two metals (Pb and Cu) produced significant increases in cell size and two other metals (Co and Ni) caused significant decrease in cell size. Mn, Pb, Co, Ni, and Cd caused significant increases in the surface area of the cell's thylakoids and Ni, Co, Zn, and Hg caused a significant reduction in the volume of the intrathylakoidal spaces. All tested metals except Pb and Mn caused the production of extra intracellular membrane whorls. Pb, Mn, Co, Zn, Hg, Cd, and Ni caused coalescence of cellular lipid while Cu caused a reduction in cellular lipid content. Co caused a reduction in the number of polyhedral bodies and Cd caused an increase in the volume of the cell's polyphosphate bodies. The physiological significance of these findings are discussed in terms of potential toxic action of cations and the cellular mechanisms for the detoxification of cations once they enter the cell. Common mechanisms of both procaryotic and eucaryotic algae are discussed and the utilization of stereological techniques for electron microscopic morphometric analysis is established in studies of metal toxicity at the cellular level.