The sensitivities and response times of six ventilators in the assist mode were measured. The withdrawal volumes necessary to cause triggering were: Bennett MA-1, 0.5 ml; Bennett PR-2, 1.5 ml; Bird Mark 8, 1.5 ml; Bourns LS 104–150, 0.1 ml; Dräger Spiromat 661, 1.0 ml; Ohio 560, 0.5 ml. The times necessary for the ventilators to respond to these withdrawals were: Bennett MA-1, 82 msec; Bennett PR-2, 226 msec; Bird Mark 8, 157 msec; Bourns, 36 msec; Dräger, 434 msec; Ohio, 123 msec. The maximal rate at which a ventilator can assist is inversely proportional to the response time.