Objective. Aimed to describe the characteristics of the onset of puberty in males in Colombian hair ram lambs (Ovino de Pelo Colombiano, OPC), and their crosses with Katahdin and Santa Inés sheep in a farm located in Villavicencio, Meta. Materials and methods. 15 lambs of three biotypes: OPC x OPC, Santa Ines x OPC (SO) and Katahdin X OPC (KO) from four months of age, five per group. Ram lambs were grazing and they had supplementation with commercial salt and water ad libitum. Monthly body weight, testicular measurement, scrotal circumference (CE) were measured and serum testosterone were determined by Elisa test, an ultrasound of the two testicles was performed to determine the presence of the testicular mediastinum, also evaluating the presence and detachment of the urethral prolongation and then electro ejaculation was performed to determine the macroscopic and microscopic characteristics of the semen. Results. Genotype effect was significant for evolution of body development (BW, SC, TV) over time, Santa Ines and Katahdin crosses presented the highest values in BW during the investigation whereas OPC lambs maintained a lower value until the end of the trial. At least four evaluations were made in the males or until the presentation of a minimum concentration of 150 million of sperm per ml with 30% of individual progressive motility.  Conclusions.Around six months old is the puberty onset for ram lambs under tropical conditions, in a non-seasonal country, based on a diversity of variables including body development, semen quality, and testosterone levels among others.