A facility for bidding on a pending advertising opportunity is described. The facility receives at an advertiser a notification from a publisher. The notification contains keywords relating to a content request received by the publisher, and is sent by the publisher between its receipt of the content request and its dispatch of a content response responsive to the content request. In response to receiving the notification, the facility sends to the publisher a dynamic bid. The dynamic bid specifies an advertising message and a bid amount that the advertiser is willing to pay to present the advertising message in connection with a content response responsive to the content request produced by the publisher.