A homologous radioimmunoassay (RIA) system was developed for human GRF 1-40 and used to measure immunoreactive (IR) concentrations of the peptide in rats to determine some of its pharmacokinetic characteristics after intravenous (i.v.) and subcutaneous (s.c.) administration. A plot of the disappearance of IR-hGRF from plasma after a single intravenous injection was fitted by a biexponential curve, analysis of which gave a half-life of 3.2 +/- 0.2 min for the initial distribution phase and 57.3 +/- 1.5 min for the elimination phase. Comparison of areas under the plasma IR-hGRF/time curves after injection of identical doses of hGRF 1-40 showed that the amount detected in the circulation after it was injected s.c. was only 14-16% of the amount detected after i.v. administration. Such results may indicate degradation of a substantial proportion of the dose of the peptide at the site of injection or during its transfer to plasma; this should be borne in mind when undertaking s.c. administration for clinical purposes or in assessing the effect of GRF analogues.