This paper examines the impact of the Eurosystem's increased footprint in financial markets, resulting from the response to the Covid-19 crisis, on repo rates. We exploit transaction-level data on the repo market and the Eurosystem's purchase programmes and find that both marginal purchases (flow effect) and aggregate holdings (stock effect) have a significant downward impact on repo rates. The stock effect is nonlinear, and amplified when the central bank's holdings are larger. Finally, we find that the Eurosystem's Securities Lending Facility alleviates this collateral scarcity, but it does not fully compensate for the downward pressure on rates created by purchases.