In Experiment 1 hourly blood samples were collected via jugular cannula from four puberal Angus bulls during 24 h preinfusion, 12 h infusion and 12 h postinfusion periods. Episodic fluctuations in peripheral concentrations of E, LH, T, and GC were observed during the 24 h preinfusion period; however, the only significant temporal relationship observed was between concentrations of LH at 1 h and concentrations of T at the subsequent hour (LAG-LH-T-CORR, r = 0.42). Infusion of E₂ at the rate of 41.7 mg/h resulted in acute increases in circulating concentrations and areas beneath hormone profile curves for LH, T, and CC. During the postinfusion period, both LH and T areas were less than areas calculated for similar times during the preinfusion period. In contrast, GC areas were greater (P<0.10) during the postinfusion period than at similar times prior to infusion.

Based on results of the first experiment, Experiment 2 was designed to compare the effects of E₂ infusion on circulating levels of LH, T, and GC between eight control and seven E₂-treated bulls. Blood samples were collected every 30 min for a 6 h preinfusion period after which E₂ or propylene glycol was continuously infused for 6 h at a rate of 16.7 mg/h. Samples were also collected for a maximum of 6 h postinfusion. Significant acute increases in circulating concentrations and areas of LH and T were observed in five of seven E₂-treated bulls. There were no changes in hormone levels of control bulls during infusion. Both LH and T areas were not significantly different between E₂-treated and control bulls during the postinfusion period. Infusion of E₂ did not influence levels of circulating GC and areas of GC.

These results demonstrate that E can be included with LH, T, and GC as hormones whose peripheral concentrations fluctuate in an episodic manner. Also, administration of pharmacological levels of E₂ can result in an acute increase in levels of circulating LH and T.