A new MIRD dynamic model has been used to provide estimates of the dose to the urinary bladder resulting from the administration of the therapeutic agents 131I as iodide (for thyroid carcinoma) and 131I meta-iodobenzylguanidine (MIBG) (for neuroendocrine tumours). Because the latter agent is used for therapeutic purposes in children, dose estimates were obtained for subjects aged 1 year and upwards. Those parameters likely to influence the bladder dose were also investigated, making use of the inherent flexibility of the model. For an administration of 1 GBq of either 131I as iodide or 131I MIBG to an adult subject, the radiation dose to the inner surface of the bladder was estimated to be approximately 1100 mGy, which is nearly twice the value estimated using a constant-volume bladder model. The new model produced dose estimates for children (within the range 1000-2750 mGy GBq-1 of 131I MIBG) which were approximately 50% greater than those derived using a constant-volume bladder model. The urine flow rate was found to have the greatest effect on the bladder dose, a flow of twice the normal rate resulting in a reduction in the bladder dose by a factor of two. On the other hand, a reduction in the urine flow rate to half the normal value was estimated to increase the radiation dose by a factor of two. This was true for subjects of all ages. With normal voiding, the average dose to the bladder wall from 131I beta-radiation was estimated to be 5-13% of the surface beta dose for subjects of different ages, the values being greater in children. This has to be compared with a photon contribution to the average bladder wall dose amounting to 10-20% of the combined surface dose both from beta-particles and from photons.