Theories of formal discourse representation (e.g. Ariel 1990, Grosz et al 1995) have proposed that assignment of reference to a pronominal element in ambiguous contexts, follows a rule-guided ranking of the candidate NP's. The purpose of this paper is to experimentally evaluate this proposal regarding one such ranking rule, to which we will refer as the subject rule. Both Ariel's (1990) theory of accessibility and Centering Theory (Grosz et al 1995) state that an NP in a subject position is ranked higher, all things being equal, than an NP in the object (or any other syntactic) position. Using an experimental comprehension task, we show that indeed, in cases of uncertainty, NP's in subject position are selected more often as the reference of a pronominal element. Furthermore, the same task was conducted with 18 preschool children and 7 aphasic patients. The results show that also children and aphasics make use of the subject rule to select between competing antecedents. These findings contribute to the conclusion that the subject rule plays a significant role in discourse reference assignment and that it is a fundamental rule which appears early in acquisition and remains intact in cases of language breakdown.