Somatostatin concentration was determined in plasma and in cerebrospinal fluid during a 24-h period in 7 male patients suffering from hydrocephalus of differing aetiologies. Blood and ventricular cerebrospinal fluid samples were taken every 2 h during the day (08.00-22.00 h) and every hour during the night (24.00-07.00 h). Simultaneously, plasma growth hormone levels were also evaluated. Plasma SRIH levels showed significant circadian variations with highest values in the daytime and lowest values during the night. Cerebrospinal fluid SRIH did not show any significant time-related circadian changes. Plasma GH levels showed the well-known circadian pattern in the majority of patients. No significant correlation was found between the plasma GH and plasma or cerebrospinal fluid SRIH values recorded during the 24-h period. Results suggest that peripheral SRIH does not play any major role in the control of the 24-h GH secretory pattern in man.