The effects of intravesical instillation of atropine (10(-6) M) and phentolamine (10(-6) M) on urodynamic parameters were investigated in patients with detrusor hyperactivity of neurogenic or non-neurogenic origin. A modified cystometric technique with slow intermittent filling was used. The reproducibility of the investigative procedure was first verified in 17 patients. Eighteen patients, 12 with detrusor hyperreflexia and 6 with detrusor instability, were then investigated with atropine and 17 patients, 11 with detrusor hyperreflexia and 6 with detrusor instability, were investigated with phentolamine. In individual patients clinically relevant improvements, such as an increased bladder capacity and a decreased detrusor pressure, were found. Five/12 neurogenic and 1/6 non-neurogenic patients responded to instillation of atropine. Five/11 neurogenic but none of the non-neurogenic patients responded to phentolamine. In the neurogenic group phentolamine caused an increase of the bladder capacity from 247 +/- 57 ml. to 378 +/- 57 ml. (p < 0.05). It is suggested that the different sensitivities to intravesical drug administration in patients with detrusor hyperreflexia and detrusor instability can be explained by pathophysiological differences and concluded that intravesical instillation of drugs may be of therapeutic value in selected groups of patients.