Eighteen Landrace pigs and 12 Göttingen mini-pigs were evaluated in a study of experimental bladder outflow obstruction. Twenty-two of the animals underwent partial bladder outflow obstruction for periods up to 12 months. The subsequent changes were assessed using cystometric, physiological and morphological means. There was a consistent increase in the voiding pressures and a concomitant reduction in the flow rates in all the obstructed animals. Seventy-seven per cent of the obstructed animals showed cystometric evidence of bladder instability. In vitro studies showed an increase in sensitivity to exogenously applied agonists and a reduction in sensitivity to intramural nerve stimulation. Morphological studies showed an inverse correlation between neuronal density and the duration of obstruction. These changes are typical of post-junctional supersensitivity secondary to partial denervation. These results suggest that agents capable of stabilising the bladder smooth muscle membrane may be useful in the treatment of detrusor instability secondary to bladder outflow obstruction.