[The sexual activity of male rats which had not been engaged in heterosexual activity was compared with that of others having a certain amount of sexual experience. The development of the pattern of behaviour in the later group was followed from 105 days of age to 138. It was found that: 1. Continuous changes in the sexual behaviour occurred, the frequency of ejaculations per hour increased, the number of intromissions before ejaculation decreased, the duration of the series of copulations preceding an ejaculation was shortened and the post-ejaculatory latency was shortened. 2. Comparing experienced with non-experienced rats of the same age it became evident that learning intervened only determining the lengths of the post-ejaculatory latencies the other components being unaffected. It ís concluded that individual experience affects the length of the post-ejaculatory latency, other changes appearing in the pattern of behaviour with increasing age dependent on maturing processes in the biological organism., The sexual activity of male rats which had not been engaged in heterosexual activity was compared with that of others having a certain amount of sexual experience. The development of the pattern of behaviour in the later group was followed from 105 days of age to 138. It was found that: 1. Continuous changes in the sexual behaviour occurred, the frequency of ejaculations per hour increased, the number of intromissions before ejaculation decreased, the duration of the series of copulations preceding an ejaculation was shortened and the post-ejaculatory latency was shortened. 2. Comparing experienced with non-experienced rats of the same age it became evident that learning intervened only determining the lengths of the post-ejaculatory latencies the other components being unaffected. It ís concluded that individual experience affects the length of the post-ejaculatory latency, other changes appearing in the pattern of behaviour with increasing age dependent on maturing processes in the biological organism.]