During pregnancy a number of continuously changing circumstances exist which might be expected to modify the relation between plasma drug levels and drug dosage. Alimentary tract motility may be decreased, the distribution of many drugs may be altered, glomerular filtration rate is greater and biotransformation capacity may be changed as pregnancy advances. However, relatively little has been published on the monitoring of plasma drug levels during pregnancy.It has been established that, in the presence of constant drug doses, plasma levels of phenytoin, phenobarbitone and certain other anticonvulsants tend to fall during pregnancy and rise again during the puerperium. Plasma lithium and possibly digoxin levels also fall relative to drug dose as pregnancy progresses, and rise again in the puerperium.While the changes in lithium and digoxin levels are probably chiefly due to increased rate of glomerular filtration during pregnancy, the altered anticonvulsant requirement is more likely to depend mainly on an increased rate of biotransformation. Anticonvulsant plasma levels should be monitored regularly from the outset of pregnancy and more frequently after birth.