PIP: Prior to considering the effects of herbs and drugs during pregnancy and lactation, attention is directed to the relevance of herbal medicines to human patients. When a drug is taken orally, its absorption, biotransformation, and excretion will affect its blood level. It then makes its way to the capillaries surrounding the alveoli of the breast, and the drug must cross the endothelium of the capillaries into the alveolar cells of the breast and then be excreted into the lumen with the milk. Most of the drugs are excreted into milk by a process of diffusion as a result of a variation in the concentration gradient between plasma and milk. Transfer of drugs also depends on their molecular size. A table of drugs which have been found to be excreted or not, and if so whether the levels would be significant for the infant or not, is presented. Several years of observation of prescription habits of herbalists in Singapore have brought to the surface the use of certain herbs which form the core of different prescriptions. The following "core" herbs and their variations are described: radix angelicae sinensis; radix rehmanniae; radix poeoniae alba; rhizoma ligusticum wallichii; rhizoma atractylis macrocephacae; glycyrrhiza radiz; ginseng radiz; gelatinum assini; radix codonopsis philosula; radix astragali; rhizoma cyperi; cinnamonium cassia; fritillaria cirrhosa; and poncirus trifoliata. These "core" herbs are used in various presentations and combinations in various amounts in certain prescriptions for certain purposes during pregnancy. It is difficult to confirm the effects which the Chinese herbalists claim when these herbs are prescribed for pregnant women. The conclusion is that there is no specific effect on the pregnant woman, but this does not exclude the possibility of a beneficial psychosomatic effect. It must also not be forgotten that the acttive principles can cross the placenta and reach the fetus.