Within 30 min of starting continuous ivy infusion of 333 mug thyrotropin-releasing hormone (TRH)/hr into 12 cows, serum prolactin (PRL) increased more than 10-fold and growth hormone (GH) increased 2.6- to 4-fold above basal concentrations. Constant infusions of 30 mg/hr of prostaglandin F2alpha (PGF2alpha) increased serum PRL and GH to maxima wius infusion of TRH or PGF2alpha, PRL declined throughout the 6- to 13-hr infusion interval although it remained well above preinfusion or saline-infusion control values. Serum GH declined more rapidly than PRL in the face of TRH or PGF2alpha infusions, reaching basal concentrations in one experiment within 1-3 hr. Application of milking stimuli during the 5th hr of TRH infusion caused an additional increase of 23 ng/ml of PRL above the TRH-stimulated concentrations. Similarly, intravenous injection of 5 mg PGF2alpha during the 5th or 12th hr of TRH infusion increased serum PRL an additional 582-682 ng/ml and further increased serum GH 9-91 NG/ML. When 200 mug TRH was injected during the 5th hr of a PGF2alpha infusion, serum PRL increased another 267 ng/ml and GH increased an additional 61 ng/ml. Administration of 10 doses of TRH of 200 mug each in 2 hr did not increase PRL or GH in the serum above that observed when 2.2 mg TRH was infused over a 6-hr period. Collectively, the data suggest that a ceiling exists in cows for secretion of PRL and GH, but this ceiling may be overcome with application of a second heterologous stimulus.