In this study, we tested the paired-pulse transcranial magnetic stimulation (ppTMS) paradigm - a conditioning stimulus (CS) given at variable intervals prior to a test stimulus (TS) - for visually evoked single unit activity in cat primary visual cortex. Here, we defined the TS as being supra-threshold when it caused a significant increase or decrease in the visually evoked activity. By systematically varying the inter-stimulus interval (ISI) between 2 and 30 ms and the strength of CS within a range of 15-130% of TS, we found a clear dependence of the ppTMS effect on CS strength but little relation to ISI. CS effect was strongest with ISI of 3 ms and steadily declined for longer ISI. A switch from enhancement of intra-cortical inhibition at short ISI (2-5 ms, SICI) to intra-cortical facilitation (ICF) at longer ISI (7-30 ms), as demonstrated for human motor cortex, was not evident. Whether the CS caused facilitation or suppression of the TS effect mainly depended on the strength of CS and the polarity of the TS effect: Within a range of 60-130% a positive correlation between ppTMS and TS effect was evident, resulting in a stronger facilitation if the TS caused facilitation of visual activity, and more suppression if the TS was suppressive by itself. The correlation inverted when CS was reduced down to 15-30%. The ppTMS effect was not simply the sum of CS and TS effect, it was much smaller at weak CS strength (15-50%) but stronger than the sum of CS and TS effects at CS strength 60-100%. Differences in the physiological state between sensory and motor cortices and the interactions of paired synaptic inputs are discussed as possible reasons for the in part different effects of ppTMS in cat visual cortex and human motor cortex.