Despite considerable evidence suggesting that second language (L2) learners experience difficulties when processing morphosyntactic aspects of L2 in online tasks, the mechanisms underlying these difficulties remain unknown. The aim of this dissertation is to explore possible causes for the difficulties by comparing attentional mechanisms engaged at the early stage of lexical access in native and nonnative language processing. The study utilized a grammatical priming paradigm to examine the manner in which native and L2 speakers of Russian access and integrate morphosyntactic information when processing gender and number agreement that operates between nouns and adjectives within the same noun phrase (e.g., "prostoj kozjol" "simple-MASC-SG goat-MASC-SG") and between nouns and verbs across phrasal boundaries (e.g., "byl kozjol" "was-MASC-SG goat-MASC-SG"). While native participants (N = 36) invoked both automatic and strategic attentional mechanisms, highly proficient L2 participants (N = 36), who had been able to perform at the native-like level in offline tasks, exhibited delayed activation of morphosyntactic information and reliance on strategic mechanisms that operate after lexical access. The finding suggests that L2 difficulties with grammar, that are usually regarded as deficits, may reflect differences in the dynamics of lexical activation. The study also found robust priming effects for both categories and evidence of the Markedness Effect (Akhutina et al, 1999) in both groups of participants: nonnative participants recorded differences in the magnitude of priming between feminines and masculines as well as between singulars and plurals, and native participants showed differential contribution of facilitatory and inhibitory components of priming in response to different genders and numbers. The findings suggest that gender and number may require different processing mechanisms, which, along with salience of morphological markers and agreement structures, may contribute to agreement processing in local dependencies more than syntactic distance. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]