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Biological productivity of grazing livestock under semi-arid conditions in northern Kenya is limited by marked seasonal fluctuations in the availability and quality of rangeland resources. With the increasing trend towards sedentarization of pastoral herds and households, the search for an alternative to herd mobility as the most effective adaptive management strategy to attenuate the effects of seasonality in production resources is an issue of growing concern. Using data collected during an experimental study conducted in pastoral goats in northern Kenya, this paper is concerned with the assessment of the effects of controlled seasonal breeding on biological herd productivity. The experiment was conducted over a period of 4 years (1984-1988) in Isiolo District, northern Kenya. The experimental treatment consisted of six different mating seasons per year, which were replicated three times over the course of the experiment. Assessment of biological productivity was done with a previously developed procedure for steady-state productivity assessments in livestock herds. The criterion used to evaluate biological herd productivity was an energetic efficiency ratio, defined as the total gross energy output of the herd divided by its total metabolizable feed energy requirements at the stationary state. The first hypothesis for testing was that there is an optimal period in a year to which breeding can be restricted to improve overall biological productivity of pastoral goat flocks. Second, a simulated, aseasonally reproducing herd was used as a reference in testing whether controlled breeding is superior to uncontrolled breeding. The general conclusion that emerged from this study is that restricted breeding can effectively be used as a management control to manipulate overall biological herd productivity primarily because of its positive effect on youngstock mortality rates. For achieving maximum energetic efficiency in pastoral goat flocks, breeding should be constrained to the long dry season (June to October). However, when compared to this breeding regime simulated aseasonal breeding performed remarkably well, and more field research is required to ascertain the superiority of controlled over aseasonal breeding.