Brief episodes of apparently unprovoked tachycardia (increased heart rate, HR) lasting about 10–20 min were observed about one h after initiation of experimentation in 10 ovariectomized ewes which were fitted with indwelling vascular cannulae. The incidence of tachycardia episodes varied among ewes, but 99 of 215 (46%) sampling trials contained such an episode (mean HR increase = 34 beats/min). Approximately 30–40 min after maximum tachycardia when HR had returned to basal rates, both plasma prolactin (Prl) and growth hormone (GH) increased significantly from the nadir concentrations that had been observed around the time of maximum tachycardia and shortly thereafter. There were no changes in packed cell volume (PCV) and only brief changes in mean arterial blood pressure related to the tachycardia. There was a mild trembling behavior associated with many of the tachycardia episodes. The causes for the tachycardia are not known. Heparin administration did not seem to be involved, but physical stimulation inside the jugular vein, while not essential, may increase the incidence of tachycardia episo