As postpartum depression can occur in up to 10% of mothers, antidepressants will often be required in mothers who are breast-feeding. The judicious use of antidepressants is important since all psychotropic drugs cross into the breast milk. Drug excretion into breast milk occurs primarily by passive diffusion. Small, highly lipid soluble, unionised drugs diffuse more rapidly than other drugs. It must be remembered, however, that for those drugs with a high volume of distribution, such as the highly lipid soluble drugs, most of the drug is outside the plasma compartment, leaving only a small proportion free to transport from plasma to milk. Also, most lipophilic drugs are concentrated in hind milk which is richer in fat than fore milk. All of the antidepressants are highly lipid soluble with large volumes of distribution.