Parkinson's disease (PD) is known to affect retinal structure and activity. As such, retinal evaluations may be used to develop objective and possibly early PD diagnostic tools. The aim of this study was to investigate the effects of Parkinson's disease (PD) manifestation and treatment on retinal activity. Data were collected on 21 participants diagnosed with PD, including the number of medications taken, clinical scales and flash electroretinography (fERG) measurements, under light-adapted and dark-adapted conditions. The fERG parameters measured included a-wave and b-wave amplitude and implicit time (i.e., latency). First, we investigated correlations between symptom measure scores and the fERG parameters. Next, we divided participants into two groups based on their antiparkinsonian medication load and analyzed differences between these groups' fERG parameters. fERG parameters were strongly correlated with a number of clinical variables, including motor and non-motor symptoms and age at PD onset. Photoreceptor cell implicit time was longer among participants taking one or less antiparkinsonian medication as compared to those taking two or more. However, overall there was not strong evidence of a relationship between the number of antiparkinsonian medications taken and the fERG parameters. Findings suggest that fERG may be a useful, non-intrusive measure of retinal, and, perhaps overall CNS function, in PD. However, additional studies in larger samples are needed to clarify this association.