Biphasic reaction on blood pressure is seen With the administration of:

A moderate dose of adrenaline given by I.V produces a rapid increase in BP followed by a fall - a biphasic response. The systolic rises due to alpha 1 mediated vasoconstriction and beta 1 mediated increase in hea rate, the force of contraction and thereby cardiac output. Action on beta receptors is more persistent and as the action on alpha receptor wears off the action of beta 2 receptors gets unmasked resulting in decreased BP due to a fall in diastolic. From medical pharmacology Padmaja 4th edition page no 94,95