In 8 children with cystometric evidence of bladder instability and marked systemic side effects to oral oxybutynin we investigated the efficacy of intravesical instillation and compared the pharmacokinetics of both routes of delivery. In addition, 4 healthy dogs underwent intravesical instillation for pharmacokinetic studies. Intravesical oxybutynin was well tolerated, efficacious and rapidly absorbed, resulting in plasma concentrations markedly higher than after oral administration. In only 2 patients did this method of instillation result in side effects, and both had previously undergone bladder augmentation. This lack of significant systemic side effects despite high plasma concentrations suggests that a metabolite may be generated after oral administration that is responsible for the side effects. These studies demonstrate that the mode of administration affects the mechanism of action, side effects, pharmacokinetics and metabolism of oxybutynin, and that intravesical instillation is clinically effective and results in minimal side effects.