Title: Membership Leakage in Pre-trained Language Models. Abstract: Pre-trained language models are becoming a dominating component in NLP domain and have achieved state-of-the-art in various downstream tasks. Recent research has shown that language models are vulnerable to privacy leakage of their training data, such as text extraction and membership leakage. However, existing works against NLP applications mainly focus on the privacy leakage of text generation and downstream classification, and the privacy leakage of pre-trained language models is largely unexplored. In this paper, we take the first step toward systematically auditing the privacy risks of pre-trained language models through the lens of membership leakage. In particular, we focus on membership leakage of pre-training data in the exposure of downstream models adapted from pre-trained language models. We conduct extensive experiments on a variety of pre-trained model architectures and different types of downstream tasks. Our empirical evaluations demonstrate that membership leakage of pre-trained language models exists even when only the downstream model output is exposed, thereby posing a more severe risk than previously thought. We further conduct sophisticated ablation studies to analyze the relationship between membership leakage of pre-trained models and the characteristic of downstream tasks, which can guide developers or researchers to be vigilant about the vulnerability of pre-trained language models. Lastly, we explore possible defenses against membership leakage of PLMs and propose two promising defenses based on empirical evaluations.