Abstract Estrogen (E 2) can modulate a variety of functional processes, including conditioning. However, the precise relationship between E 2 and these processes is not entirely understood. Indeed, the nature of E 2's effects on conditioning may depend upon several factors, including, but not limited to, the task examined, route of E 2 administration, bioavailability of E 2 administered, and/or duration of E 2 exposure. The present studies examined the effects of E 2 on conditioned place preference (CPP), and E 2 levels produced in plasma and the nucleus accumbens. In Experiment 1, ovariectomized, Long–Evans rats were subcutaneously (SC) administered sesame oil vehicle ( n = 12), 10 μg ( n = 12), or 1 mg ( n = 10), E 2 immediately prior to placement in the CPP apparatus on conditioning days. Only rats administered 10 μg E 2 exhibited a CPP. This regimen of E 2 ( n = 5/group) also produced moderate levels of E 2 in the nucleus accumbens (significantly greater than vehicle and less than 1 mg E 2). In Experiment 2, ovariectomized rats were SC administered propylene glycol vehicle ( n = 11), 10 μg ( n = 13), or 1 mg ( n = 14), E 2 immediately prior to conditioning. Administration of 1 mg E 2 in propylene glycol produced a CPP. Notably, 1 mg E 2 in propylene glycol produced moderate levels of E 2 in the nucleus accumbens (significantly greater than vehicle or 10 μg E 2) that were similar to those produced by 10 μg E 2 in sesame oil ( n = 5/group). Together, these data suggest that regimen of E 2 that can produce a CPP result in moderate levels of E 2 in the nucleus accumbens.