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The present work aimed to investigate the effect of nutrition supplementation on skin follicle activity and wool growth rate in male and female Sanjabi lambs during the autumn and winter seasons, when the natural daylength is declining and being shortest. Twenty male (control and male (CM), n = 10; treatment and male (TM), n = 10) and 20 female (control and female (CF), n = 10; treatment and female (TF), n = 10) Sanjabi lambs were housed in individual pens, under natural daylength condition at west of Iran, Kermanshah (34°18′ N and 47°3′ E and 1420 m above sea level). Lambs in control groups received a diet consisting of 80% alfalfa and 20% concentrate, providing 2.18 Mcal and 130.0 g per kg DM ME energy and crude protein, respectively. In treatment groups, lambs were fed a diet consisting of 65% alfalfa and 35% concentrate, providing 2.34 Mcal and 160.0 g per kg DM ME energy and crude protein, respectively. Raw and clean fiber growth rates and fiber diameter were measured from left mid-side patches harvested at the end of every month. Percentage of active primary and secondary follicles (PAP and PAS), follicular density (FD) and the ratio of secondary to primary follicles ( S / P ) were measured from skin biopsies, taken from the right mid-side of the lambs in monthly intervals. PAP and PAS in autumn were significantly ( p < 0.01) higher than those observed in winter season. There was a positive effect of feeding supplementation on both PAP and PAS but, similar values for S / P ratio and FD were observed in control and treatment groups. All hair follicle parameters viz. PAP, PAS, S / P ratios and FD were significantly greater in females compared to male lambs. The rate of wool growth was significantly lower in winter compared to autumn season, although similar fiber diameter was recorded in two seasons. Male lambs compared to females had greater body weight, DMI and the rate of wool growth. It is concluded that the fiber follicle activity and therefore wool growth in Sanjabi lambs is mainly under influence of nutrition, nonetheless little monthly fluctuation in follicle activity and fiber growth and diameter was observed. Likewise, there was a different pattern of wool production in male and female Sanjabi lambs. [ABSTRACT FROM AUTHOR]