A flicker sensitivity test based on measuring modulation thresholds for light varying sinusoidally with time around 2 mean scotopic luminances was performed on 21 female patients heterozygous for X-linked retinitis pigmentosa and 22 normal observers. The patients required, on average, 1.5 times the normal modulation to detect flicker over the whole frequency range where thresholds could be measured. Apparently the rod system of heterozygotes exhibits a reduced signal-to-noise ratio compared with normal but that the dominant time delays in the signaling process are not significantly altered.