1. Cyclo-oxygenase (COX)-2 inhibitors and other non-steroidal anti-inflammatory drugs (NSAIDs) have been implicated in increased cardiovascular events. However, the direct effects of these drugs on cardiac function have not been explored extensively. Given the important role of the renin-angiotensin-aldosterone system (RAAS) in cardiac remodelling, we sought to determine the effect of COX-2 inhibitors and non-specific (NS-) NSAIDs on RAAS-induced cardiac hypertrophy and fibrosis in neonatal rat cardiac myocytes (NCM) and fibroblasts (NCF) isolated from 1-2-day-old Sprague-Dawley rat pups. 2. The NCM were pretreated for 2 h with COX-2 inhibitors (celecoxib or rofecoxib) or NS-NSAIDs (naproxen; all at 0.1-10 micromol/L) before being stimulated with 10 micromol/L aldosterone for 72 h or with 0.1 micromol/L angiotensin (Ang) II for 60 h. Hypertrophy of NCM was assessed by [3H]-leucine incorporation. 3. The NCF were pretreated with COX-2 inhibitors or naproxen as described for NCM before being stimulated with 0.1 micromol/L AngII for 48 h. Collagen synthesis was subsequently assayed by [3H]-proline incorporation. 4. Pooled cryopreserved male and female rat hepatocytes were treated with or without COX-2 inhibitors for 1 h before 1 nmol/L aldosterone ( approximately 540 pg/mL) was added to all wells. Cells were incubated for a further 60 min and culture media harvested by centrifugation. Human hepatic HepG2 cells were treated with compounds with or without serum starvation for 48 h. All cells were pretreated with COX-2 inhibitors for 2 h before the addition of aldosterone. Cell culture media were harvested after a further 3, 18, 24 or 48 h incubation. Aldosterone concentrations in the culture media were determined by enzyme immunoassay. 5. Aldosterone- and AngII-stimulated NCM hypertrophy was inhibited by celecoxib, but not by rofecoxib or naproxen. In NCF, AngII-stimulated collagen synthesis was inhibited by celecoxib and, to a lesser extent, by rofecoxib, whereas naproxen had no effect. The COX-2 inhibitors inhibited aldosterone uptake and/or metabolism by rat hepatocytes, but had no effect in human hepatic HepG2 cells. 6. These results demonstrate a potential antiremodelling effect of selective COX-2 inhibitors in the setting of RAAS stimulation in cardiac cells, whereas naproxen has no effect.