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The concentrations of peripheral plasma testosterone were measured by radioimmunoassay in samples collected from five bulls, each given i.m. injections of 0, 5, 15, 30 and 60 mg prostaglandin F₂ (PGF₂). Synchronized peaks in testosterone concentration occurred with maximum values 1–2 h after treatment. These increases of testosterone persisted significantly longer than those observed to occur as natural episodic peaks during two 24 h periods in the same bulls. The mean peak testosterone concentration after PGF₂ injection was related to the dose of PGF₂, values after 60 and 30 mg doses being significantly greater than after 15 mg. The response produced by a 5 mg dose was not significant. The results indicate that intramuscular injection of PGF₂ acutely stimulates synthesis and release of testosterone in bulls.