The beta-adrenoceptor antagonist Timolol Maleate (0.5%) was employed topically to determine whether the resting (or "tonic") position of accommodation is a consequence of steady-state equilibrium between parasympathetic and sympathetic innervation of the ciliary muscle. Ten subjects were used, eight men and two women, with an age range of 19-37 (mean = 23.2). All subjects wore an optimum refractive correction giving visual acuities of 20/20 or better. Darkroom measurements of tonic accommodation (TA), tonic vergence (TV), and pupil diameter were determined with a Badal laser optometer , a nonius alignment device and infra-red photography, respectively. Far and near points of accommodation and oculo-motor balance tests were determined by standard optometric techniques and intraocular pressure (IOP) was measured by applanation tonometry. Both TA (mean = 1.66 D) and TV (mean = 2.10 deg) assumed intermediate resting positions in the absence of visual stimulation. A double-blind protocol between Timolol and Saline demonstrated a mean myopic shift in TA of +0.85 D over 40 min with Timolol. All other experimental parameters, apart from IOP, were unaffected by Timolol. It is concluded that sympathetic innervation to the ciliary muscle plays a significant role in determining the TA position and that this innervation is mediated by inhibitory beta adrenoceptors.