Local injection of hormones into the tibial epiphyseal growth plate offers a possible model to answer whether sex steroids can affect bone growth directly. To answer this question, we injected different doses of testosterone enanthate (4, 40, 120 and 400 micrograms/100 g of rat weight) once into the tibial epiphyseal growth plate of castrated 35-day-old male rats. The contralateral tibia was injected with sesame oil and served as control. All animals were sacrificed at age 42 days. Tibias were removed for measurement of epiphyseal growth plate width and blood was collected for measurement of serum IGF-I and testosterone. The lower doses of testosterone enanthate (4, 40 and 120 micrograms/100 g) did not produce any significant change in epiphyseal growth plate width. Testosterone at the largest dose tested (400 micrograms/100 g) increased epiphyseal growth plate width by about 15% compared to control (p less than 0.01). At this dose, serum testosterone was not increased, suggesting that the effect on epiphyseal growth plate width was not due to higher systemic testosterone concentrations. No differences in IGF-I levels were observed among the groups. We conclude that direct administration of testosterone enanthate at a dose of 400 micrograms/100 g into the rat tibial epiphyseal growth plate can increase epiphyseal growth plate width.