The reason for this may lie in the structural characteristics of these markets rather than in a failure of tactics or of coordination of law enforcement efforts. The Federal effort has aimed at importation and high level distribution, which account for a modest share of the retail prices of these drugs. Increasing risks to importers and high level distributors, thus, is likely to have only a modest effect on retail prices and is unlikely to have any effect on the conditions of use. Street-level enforcement is hindered by the sheer scale of the two markets and because so few of the final purchases occur in public settings. Many of the risks associated with drug trafficking come from the actions of participants in the trades themselves, and this also limits the ability of law enforcement agencies to act in ways that will cause price increases or alterations in market conditions. Compared to marijuana and cocaine markets, the heroin market is much smaller and demand is far more likely to be elastic to price. Both of these factors increase the likely efficacy of enforcement efforts and suggest that it is toward this market that funding, investigation, and prosecution ought be directed. 36 references. (Author abstract modified)