In conscious dogs equipped with ameroid constrictors to produce gradual coronary occlusion, coronary flow velocity was monitored prior to complete occlusion when coronary constriction was severe (resting flow velocity reduced by 10-50% from control recordings made days after ameroid implantation). In six of the ten dogs, we observed spontaneous cyclical variations in coronary flow velocity, characterized by gradual reduction in flow followed by very abrupt restoration of flow. The cyclic coronary flow reductions were observed between 20 and 31 days after ameroid implantation. These changes in flow bear striking similarity to those observed by previous investigators using anesthetized, open-chest canine preparations, in which the role of platelets was clearly demonstrated. Consequently, we hypothesize that spontaneous platelet aggregation and de-aggregation within the severely narrowed coronary lumen (enclosed by the ameroid constrictors) could account for our observations.