Different dosages of ¹²⁵I-toluidine blue and ⁷⁵Se-selenomethionine were injected intravenously into rabbits, and the concentration of radioactivity was measured in the parathyroids and surrounding tissues. The response of i.v. injected glucagon was also determined. It was found that 0.16 mg ¹²⁵I-toluidine blue gave a concentration ratio of parathyroid/thyroid and parathyroid/muscle which was significantly higher than both lower and higher doses at 1 minute after i.v. injection of the labelled dye. Glucagon had little or no effect on the uptake of ¹²⁵I-toluidine blue and ⁷⁵Se-selenomethionine.