Grammaticality judgment tests (GJTs) have been, and continue to be, frequently used in the field of SLA as a measure of learners' linguistic ability in the second language (L2). However, only a few studies have examined their construct validity as measures of implicit and explicit knowledge (Bowles, 2011; R. Ellis, 2005), and even fewer have explored in detail how features of these tests, such as time pressure and task stimulus, affect their construct validity (Loewen, 2009). The purpose of this paper is to examine the effect that time pressure and task stimulus have on the type of knowledge representations on which L2 learners draw when performing GJTs. The results show that the grammatical and ungrammatical sections of a timed and an untimed GJT loaded differently in both exploratory and confirmatory factor analyses. This finding can be interpreted as indicating that grammatical and ungrammatical sentences constitute measures of implicit and explicit knowledge, respectively. Additionally, the results show that time pressure and task stimulus have significant effects on learners' performance on GJTs.