We construct a tractable general equilibrium model of DeFi lending to shed light on the role of pricing rules. We determine how the rule controls key equilibrium variables such as the utilization rate. Our model delivers a measure of welfare which incorporates the DeFi borrowing rate and the security of the underlying (Proof-of-Stake) blockchain, which we use to find welfare-maximizing pricing rules. Using a genuine function of the utilization rate becomes meaningful when there is parameter uncertainty. We establish conditions under which the first-best can be implemented by such a function, which we exhibit explicitly. When these conditions are not met, allowing the rule to also depend on the staking level restores efficiency. Our analysis leads to several other practical recommendations and conceptual clarifications.