Background: Spending on pharmaceuticals is increasing faster than the gross national product in western European countries. The use of generic drugs may result in direct drug cost-savings for patients, governments and insurers. Objective: The objective of this study was to examine the drug cost development of the two most commonly dispensed antidepressant drugs in Finland following the introduction of regulatory changes to mandate generic substitution by pharmacists in April 2003. Methods: The prescription claims database of the Social Insurance Institution of Finland was used to calculate the quarterly cost of reimbursements per user for citalopram and fluoxetine from January 2003 to June 2004. Price data for the originator brand of citalopram, market incumbent brand of fluoxetine, and generic versions of both citalopram and fluoxetine were extracted from the registry of the Association of Finnish Pharmacies for April 2003 to April 2004. Results: The quarterly drug cost of citalopram and fluoxetine per user to the Social Insurance Institution of Finland decreased by 45% and 48%, respectively, 1 year following the changes to the regulations regarding generic substitution. The companies that marketed the originator brand of citalopram and the market incumbent brand of fluoxetine adopted two contrasting pricing policies in response to changes in the regulations regarding generic substitution. A 'pseudo-generic' version of citalopram, but not fluoxetine, was available throughout the study period. Regardless of the pricing policy, equivalent drug cost-savings were realised for both patients andgovernment. These cost reductions did not lead to a corresponding increase in the use of citalopram or fluoxetine. Conclusions: The introduction of changes to regulations regarding generic substitution was followed by adecrease in price for the two most commonly dispensed antidepressant drugs in Finland. High price elasticity was observed within but not between active ingredients. Price did not appear to be an important factor guiding antidepressant prescribing by physicians.