Cardiac function was measured in 16 prepubertal Ecuadorean patients with growth hormone receptor deficiency given insulin-like growth factor I (IGF-I) during part of a clinical trial. The IGF-I was given subcutaneously twice daily at a dose of 40 micrograms/kg on days 1 and 2, 80 micrograms/kg on days 3 and 4, and 120 micrograms/kg thereafter. Heart rate was determined at baseline (pretreatment) and on days 1-7 by repeated palpation of the radial artery and at baseline and on days 2, 4 and 7 by continuous portable Holter monitoring. Heart rate measured by both methods rose progressively with increasing doses of IGF-I. The mean palpated pulse exceeded baseline on each treatment day and was significantly higher on day 5 than day 4 and significantly higher on day 3 than day 2. The mean Holter heart rate was significantly higher on day 4 than on day 2 and significantly higher on day 2 than at baseline. Non-significant glucose and electrolyte changes did not appear to be associated with the cardiac events.