An experimental procedure which chronically reduces the lumen of the urethra in adult female rats produced distension of the bladder and conspicuous thickening of its wall, resulting within 6-8 weeks in a ten-fold increase in muscle weight (muscle hypertrophy). During this process, the neurons in the pelvic ganglion that innervate the bladder undergo a large increase in size (neuronal hypertrophy). The average neuronal volume increased by 83%; small neurons became less numerous and large neurons became more numerous than in controls, but there was no increase in the maximum neuronal size. Six weeks after re-operation and removal of the urethral obstruction, the weight of the bladder was reduced (although not quite to the control levels), while the average neuronal size reversed to values very close to controls. In separate experiments, the pelvic ganglion of one side was removed. The nerve fibres in the hemidenervated bladder sprouted, grew and spread to innervate the whole bladder. The neurons in the surviving pelvic ganglion hypertrophied, the average cell volume increasing by 50% in seven weeks. The experiments showed that: (i) the pelvic neurons of adult rats are capable of very extensive growth when the tissue they innervate (bladder muscle) undergoes hypertrophy; (ii) the neuronal hypertrophy is reversible. This was taken to imply that there are factors within the bladder, including trophic substances, that regulate nerve cell volume not only by inducing growth but also by inducing the opposite effect, a cell size reduction; (iii) unilateral ganglionectomy, which did not induce muscle hypertrophy but doubled the amount of muscle innervated by the contralateral ganglion, was followed by marked neuronal hypertrophy.