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Semantic number magnitude is represented in an analogue visual-spacial system in the shape of a left-to-right oriented, logarithmically compressed mental number line; the most important evidence is the SNARC (Spatial Numerical Association of Response Codes)-effect (Dehaene, Bossini & Giraux, 1993). In the actual study the spacial characteristics of number processing and of the mental number line were examined by use of behavioral data and functional imaging. It was of special interest, if the orientation in external space and on the mental number line relies on different or common neural circuits. Apart from effects concerning number processing like the SNARC- and the distance effect (Moyer & Landauer, 1967) we also examined effects of stimulus-response-compatibility like the Simon effect (Simon & Rudell, 1967). A magnitude comparison task with a fixed standard of 5 was performed in a combined reaction-time- and fMRI-study. Stimuli were presented on four positions on the monitor (above, below, right and left). Responses were given with the right and left hand. Both established and previously unknown interactions and effects were found, that point towards a shared network of spacial orientation both in external space and on the mental number line. A previously unknown facilitation effect of stimulus location and mental number processing was found in the sense that congruency of stimulus position in external space and semantic position on the mental number line enhanced the distance effect. Furthermore right sided presentation resulted to a stronger distance effect. Correspondingly the SNARC effect was strengthened but only with marginal significance. In both cases the spacial position of the stimulus influenced number processing. In the opposite direction, implicit spacial information evoked by number processing influenced the representation of external space. A modulation of the Simon effect (up to its reversal) was shown depending on congruency of stimulus position and position on the mental number line. Besides that an interaction of Simon- and SNARC effect was shown, in the sense of a stronger SNARC effect in Simon-compatible conditions. This finding supports the assumption, that both effects rely on overlapping networks of spacial representation as described by Notebaert and co-workers (2006). The functional imaging part of the study examined the neural correlates of the SNARC effect using fMRI, showing activation around the intraparietal sulcus (only right-sided). Apart from that no other SNARC-specific neuronal networks were found. The pattern of activations of the Simon effect corresponded with the pattern described in literature. Concerning the SNARC-Simon-interaction shown in the behavioral data, overlapping regions around the right intraparietal sulcus were found that are involved in both effects. These activations are potential correlates of the interaction found in the behavioral data. These results further support the assumption of partially overlapping parietal circuits for number processing, arithmetic and processing of spacial dimensions like size, position or eye movement.