The urinary bladder undergoes plastic changes during physiological alterations such as pregnancy. This study has shown that bladders from pregnant rats weighed three times more than bladders from virgin rats. Each milligram of detrusor muscle from pregnant rats contracted more strongly to nerve stimulation (150% greater) and agonists (50% greater or more) compared to detrusor from virgin rats at any stage during the oestrus cycle. The purinergic component of nerve-mediated responses altered during the oestrus cycle, being greatest during oestrus (oestrogen and progesterone fall rapidly) and dioestrus (low oestrogen and progesterone), smaller during pregnancy and even smaller during pro-oestrus (high oestrogen and progesterone); in contrast the cholinergic component remained relatively unchanged. In conclusion, during pregnancy the detrusor muscle generates larger contractions compared to virgin detrusor muscle, probably due to hormonal influences on smooth muscle contraction mechanisms. As agonist responses were unchanged during the oestrus cycle, changes in the purinergic component of nerve stimulation was not due to altered P2 receptor expression but possibly to an increase in ATP release or a reduction in breakdown. The hormonal effect may have implications for the treatment of bladder disorders due to alterations in hormones, such as stress incontinence in post-menopausal women.