Administration of purified porcine TRH to mice by intravenous subcutaneous, intraperitoneal, or intramuscular routes produced a significant release of TSH from the anterior pituitary gland. Subcutaneous administration of TRH in dilute acetic acid or 5% carboxymethylcellulose produced a prompt but somewhat smaller response at 2 h that increased between 2 and 4 h. Subcutaneous injections of TRH emulsified in glycerol and in sesame oil were associated with an approximately 30% decrease in response at 2 h as compared with intravenously administered TRH. TRH emulsified in Freund’s adjuvant and injected subcutaneously was associated with a 62% decrease in response at 2 h followed by marked increase at 4 h. It is suggested that Freund’s adjuvant, glycerol, and sesame oil slow the rate of absorption of TRH from the injection site. Plasma radioactivity levels, after the injection of tritiated TRH, correlate with the patterns of response seen when TRH is followed by bioassay using the same vehicle and mode of administrat