We followed changes occurring within bone tissue and marrow cells during the process of colchicine-induced ectopic bone development and its resorption inside the marrow cavity of the rat tibia. To stimulate ectopic bone formation male Wistar rats were i.p injected with 0.5 or 1 mg/kg b.w. of colchicine or with a 100 Fg intra-bone injection. Not all subjects responded to colchicine with ectopic bone formation in the marrow cavity, even among individuals belonging to the same strain. The kind of response in a given animal depended on the dose and site of colchicine administration. During 10 days of the experiment an increase in the occurrence of micronuclei in the polychromatic erythrocytes residing in the bone marrow (even 40-fold) was observed, indicating high genotoxicity of colchicine (at a dose of 1 mg/kg b.w. i.p. or 100 Fg intra-bone injection). An increase in the frequency of emperipolesis in megakaryocytes between the 4th and 8th days of the experiment was caused by the toxic action of colchicine and may indicate the labilisation of cell membranes and microtubule depolymerisation.