The cytogenetic effects of an analgesic antipyretic agent, metamizole, on bone marrow cells of Swiss albino male mice were studied. Animals were administered orally with 3.9, 7.8 and 15.6 mg of the drug in 0.5 ml quantities for only once (single dose series) and thrice at 24 hr intervals (cumulative dose series) and cytological preparations were made from bone marrow cells 24, 48 and 72 hr after treatment. Mitotic indices and chromosome anomalies including gaps, breaks, fragments, terminal deletions and polyploid nuclei were analysed statistically. A strong inhibition of mitotic frequency was observed with the highest doses in the cumulative dose series while it was apparently transient in the single dose series. Gaps were seen frequently. The drug appears to possess an extremely mild clastogenic but no perceivable c-mitotic potency.