Serum levels of total alkaline phosphatase increase during pregnancy but the presently available methods are not very precise. We used the CHEM 1 Bayer Diagnostic test in all the consecutive patients admitted to our unit between June 1 and October 16, 1990. Exclusion criteria were twin pregnancies, pruritus, liver disease or parasitosis. A multifactorial analysis was used to discriminate between general, obstetric, pathologic and drug parameters. There were 373 serum samples, 91% were obtained during the last three months of pregnancy. The mean serum alkaline phosphatase level was 68.2 U/l and 75.9 U/l during the first two trimesters respectively and rose to 126.7, 178.8 and 234 U/l during the last three months respectively. The elevation was greater in cases of vomiting during the first trimester, alpha-methyldopa intake. It was less after hypodroxyapatite intake and was positively correlated with weight gain. Age, parity, pregestational weight, neonatal weight, other pathologies and other drug intake did not affect the results. Knowledge of these physiological levels will be useful for evaluating most liver diseases occurring during pregnancy. Assay of the enzyme fractions would be ordered secondarily.