We investigated the effect of aging on vertical smooth pursuit, small-field optokinetic nystagmus (OKN), the vestibulo-ocular reflex (VOR), and visual-vestibular interactions (VVI). Telescopic spectacles were used to magnify the visual effects of head movements and accentuate the demand on VVI. Eleven normally sighted young subjects (age 30 +/- 6 y, mean +/- SD), and 9 normally sighted elderly subjects (age 70 +/- 8 y) were studied. Three types of stimulus motion were used: 1) predictable single sinusoidal frequencies from 0.8 to 3.2 Hz, 2) poorly predictable motion consisting of a sum of multiple harmonics of a 0.4 Hz fundamental, and 3) unpredictable velocity impulses. Tracking gains declined with frequency and stimulus unpredictability but were always greater for young than elderly subjects. Pursuit gain was always less than OKN gain for subjects tested under comparable conditions of frequency and velocity. In both subject groups, pursuit and OKN tracking exhibited phase lags at higher frequencies, particularly for poor predictable motion. Tracking phase lags were greater for pursuit than OKN and were consistently greater in the elderly than in young subjects. In young subjects, stimulus dimming using a 2 log neutral density filter reduced tracking gains and increased phase lags. In both young and elderly subjects, VOR gain was < 1.0 at low frequencies and increased slightly with increasing frequency. Visually enhanced VOR gain during normal vision was the ideal value of 1.0 at all frequencies in both subject groups. During wearing of telescopic spectacles, the young subjects exhibited marked gain enhancement that was greatest at lower frequencies and greater during predictable than unpredictable motion. Young subjects were able to suppress the VOR by fixation of real or imaginary targets moving with the head; this suppression was better at the lowest frequencies and for predictable motion. Phase errors were substantial for significant VVI in young subjects but were minimal in the elderly. Visual enhancement of VOR gain, as well as suppression by real and imaginary head-fixed targets, were attenuated in the elderly subjects. During rotations in which young subjects wore 4x telescopic spectacles, 2 log unit dimming of the visual environment reduced gain and increased phase lags. The deficiencies in VVI exhibited by elderly subjects are attributable both to deficits in the visual tracking component, as well as to nonvisual mechanisms, because even changes in mental set are less effective in modifying gain in older subjects.(ABSTRACT TRUNCATED AT 250 WORDS)