The effects of various factors on the adsorption of nicotinic acid onto and desorption from activated charcoal were investigated in vitro. The affinity of nicotinic acid for charcoal was poor both in acidic and neutral media. Adsorption increased with increasing charcoal:drug ratios and decreasing incubation volume:charcoal ratios. Desorption of nicotinic acid from dried drug-charcoal complexes was investigated in a Sartorius dissolution apparatus. The rate of the rapid, initial desorption depended on the pH and the amounts of charcoal and drug. As equilibrium was reached in the dissolution chamber, the release rate of nicotinic acid decreased slowly, depending mainly on the flow of the medium. Thus, nicotinic acid preadsorbed onto charcoal was released in a sustained manner under "continuous flow" conditions. However, because of the poor affinity for nicotinic acid, charcoal may not be a suitable matrix for sustained release of nicotinic acid.