The research presented in this paper was an attempt to find differences in the use of grammatical information carried by the function words in Serbian. The aim was to determine the level of word processing at which grammatical information shows its differential effects in groups of subjects who themselves differ in verbal ability. For this purpose, the psycholinguistic tasks applied were grammatically primed reading aloud and grammatically primed grammatical classification with an appropriate control of extra-linguistic factors that may have affected aforementioned tasks. Verbal aptitude was assessed in a psychometric manner, and the subjects were divided into "high verbal" and "low verbal" groups. Taking into account statistical control of extra-linguistic factors, the results indicate that groups of high verbal and low verbal subjects cannot be differentiated based on reading aloud performance. The high verbal subjects, however, were more efficient in grammatical classification than low verbal subjects. The results also indicated that the presence of grammatical information embedded in function words-primes had a stronger effect on word processing in low verbal group. Such pattern of results testify to the advantage of high verbal subjects in lexical and post lexical processing, while no differences were established in the word recognition processes. The implications of these findings were considered in terms of test construction for the assessment of verbal ability in Serbian language. .