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Peptides have been shown to cross the blood–brain barrier (BBB) as intact molecules so that they can influence the central nervous system. Peptides cross by saturable and nonsaturable mechanisms in the direction of both brain to blood and blood to brain. Passage of peptides, especially by saturable transport, has been shown to be influenced by pharmacological agents and physiological events. These findings support the view that peptides or their analogues could be useful as therapeutic agents for disorders of the central nervous system. They also suggest strategies in approaching therapeutic goals, including manipulating transport rates, targeting diseases due to altered BBB–peptide interactions, and designing analogues capable of taking advantage of such mechanisms of passage as paracellular trans-membrane diffusion and brain-to-blood transport.