Cytogenetical effects of the analgesic and antipyretic agent Paracetamol have been studied on meiotic cells of C3 H/He strain male mice for a period of five weeks after administering it at 0.625, 1.25 and 2.5 mg doses. Two series of experiments were carried out. In the single dose series the animals received the drug only once, while in the cumulative series, simulating continuous therapy, the drug was given consecutively to the same animals for three days at 24 h intervals. Adequate controls were employed. Quantitative studies have been made and statistical tests applied to the data. Univalents of auto- and sex-chromosomal types were higher during the first week in both series. There was a preponderance of autosomal univalents in all doses over the sex chromosomal ones up to the fourth week. Polyploid cells and translocations occurred with a low incidence. The implications of these observations are discussed.