The Slovenian Energy Law, adopted in 1999, has opened the internal market to competition up to 64% of the final consumption. The opening of the internal market to foreign competition is envisaged for 2003. With regard to the methodology of price regulation, the Energy Law introduces the "price-cap" regulation, which aims to give firms incentive for cost reduction. To provide information for effective price regulation, we estimated a cost frontier function on a sample of Slovenian electricity distribution utilities over the 1991-2000 period. The estimated efficient frontier could be used by Slovenian regulatory agency as a benchmark to regulate network access prices. Our results show that Slovenian distribution companies are cost inefficient. We have also proved the presence of increasing returns to scale with most utilities not achieving the minimum efficient scale. Thus, the Slovenian regulatory authority should consider how to induce mergers of small electricity distribution utilities into larger units. (C) 2002 Elsevier Ltd. All rights reserved.