BACKGROUND Previous studies have shown that mean 24-h GH concentrations determine plasma IGF-I levels in patients with acromegaly. However, we have recently shown that continuous GH infusion, mimicking the interpulse GH levels, was significantly more effective than the pulsatile GH administration at increasing IGF-I concentrations. OBJECTIVE The aim of the study was to ascertain relative roles of total GH output (24-h mean), GH pulses, and interpulse GH level in determining plasma IGF-I concentrations. METHODS We conducted a point-in-time observational inpatient study in the General Clinical Research Center at the University of Michigan. METHODS Eighteen patients with acromegaly and 19 healthy control subjects participated in the study. METHODS We performed frequent (every 10 or 20 min) blood sampling over 24 h. METHODS Before data collection, we hypothesized that interpulse nadir levels of GH would correlate with IGF-I levels in normal and acromegalic subjects. RESULTS Mean and valley levels of GH correlated with serum IGF-I levels (r(2) = 0.44 and 0.48, respectively) in normal and acromegalic patients in a log-linear fashion. The strongest correlation, however, was observed between the log of nadir GH and IGF-I concentrations (r(2) = 0.77). GH pulse mass did not significantly correlate with IGF-I (r(2) = 0.001). CONCLUSIONS Plasma IGF-I concentrations correlated with mean 24-h GH concentrations. This relationship is dependent exclusively on the basal GH levels. GH pulses do not determine plasma IGF-I concentrations.