Phenylketonuria (PKU) is caused by deficiency of phenylalanine hydroxylase, resulting in an accumulation of phenylalanine in brain tissue and cerebrospinal fluid of phenylketonuria patients. Phenylketonuria is neuropathologically characterized by neuronal cell loss, white matter abnormalities, dendritic simplification, and synaptic density reduction. The neuropathological effect may be due to the ‘toxicity’ of the high concentration of phenylalanine, while the underlying mechanism remains unclear. In this study, we found that cultured cerebral cortical neurons underwent mitochondria-mediated apoptosis when exposed to phenylalanine. We further demonstrated that phenylalanine induced RhoA activation. Phenylalanine also promoted myosin light chain (MLC) phosphorylation, which might be the result of the activation of Rho-associated kinase (ROCK). The RhoA antagonist, C3 transferase (C3), Rho-associated kinase specific inhibitor, Y-27632, and the overexpression of either dominant negative RhoA or dominant negative Rho-associated kinase inhibited phenylalanine-induced caspase-3 activation and rescued neurons from apoptosis, indicating that the RhoA/Rho-associated kinase signalling pathway plays an important role in phenylalanine-induced neuronal apoptosis.