1. The dependence of signal transfer in the lateral geniculate nucleus (LGN) on stimulus spatial frequency and contrast was investigated by comparing responses of individual X cells with their direct retinal inputs. 2. We used extracellular single-cell recording methods to isolate action potentials (LGN) and S potentials (SPs) from individual neurons in layers A and A1 of anesthetized and paralyzed cats. The stimuli were drifting sinusoidal gratings that were presented at each neuron's preferred orientation. The effects of stimulus spatial frequency and contrast on retinogeniculate signal transfer were determined by comparing the amplitude of the fundamental Fourier responses measured for a cell's action potentials (LGN) and its retinal input (SP) and calculating the transfer ratio (LGN amplitude/SP amplitude) for each stimulus condition. 3. In all units, the LGN response amplitude was lower than that of its retinal input regardless of stimulus spatial frequency. The mean transfer ratio measured at the peak spatial frequency for individual units was 0.56 +/- 0.03 (SE). For the majority of X LGN neurons, however, the efficiency of signal transfer varied considerably with stimulus spatial frequency. The average transfer ratio increased monotonically from 0.08 cycle/deg to near the high cutoff spatial frequency. 4. The effects of stimulus contrast on geniculate signal transfer were far more complex than previously reported and varied substantially between individual neurons. At low stimulus contrasts (< 10%), where all units exhibited linear response characteristics, only one third of our sample showed a monotonic decrease in transfer ratio with increasing stimulus contrast. The remaining two thirds either exhibited proportionately greater signal transfer for higher stimulus contrasts, or signal transfer remained relatively unchanged with increasing stimulus contrasts. When stimulus contrasts exceeded 10%, where response amplitude began to saturate, the transfer ratio was relatively constant in all units and independent of stimulus contrast. 5. Our results demonstrate that signal transfer from retina to visual cortex is regulated by LGN neurons in a stimulus-dependent manner, which appears to reflect the complex interactions between local membrane mechanisms and extraretinal inputs.