In the moderate climate zone duration of reproductive season in small ruminants is from mid-summer to mid-winter and, thus, the availability of food products of animal origin is seasonal. Considering that producers intend to respond to demands of market it is necessary to manage reproductive characteristics of their herd. For control of reproduction of small ruminants in use are among others also slow realeasing melatonin implants. Namely, quality of bucks ejaculate is significantly lower during non-breeding season. The irregularities in spermatozoa shape of head and tail and proportion of particular subpopulation of spermatozoa in ejaculate are associated with decreased fertility, early embryonic deaths and semen cryopreservation. More recently there is intensive research on morphometric characteristics of spermatozoa in domestic animals but in available literature there are no data regarding impact of exogenous melatonin on morphometric characteristics buck spermatozoa during breeding and non-breeding seasons. The objectives of this study were to: (1) determine and compare values of morphometric characteristic parameters of spermatozoa between control and treated groups of bucks during the experimental period (non-breeding season); (2) analyze obtained data regarding parameters of spermatozoa morphometric characteristics in studied groups of bucks by different statistical methods; (3) establish spermatozoa subpopulations in accordance to morphometric parameters of spermatozoa head and tail and detect differences in proportions of particular subpopulation between studied groups; (4) investigate relation between morphometric parameters of spermatozoa tail with proportion of spermatozoa motility in ejaculate. Bucks of French alpine breed (n=12) aged from 2 to 4 years were assigned into experimental and control groups. The experimental group comprised bucks (n=6) with 4 melatonin implants inserted by the end of March and control bucks (n=6) without melatonin implants. The samples of ejaculate were taken by artificial vagina on weekly basis from March to the end of May. Morphometric analyses were performed on 7289 spermatozoa in smears of semen stained by method after Bloom (n=144). Following morphometric parameters were determined for spermatozoa head and tail: area, diameter, minimal and maximal radius, length and width as well as following calculated parameters of spermatozoa for head shape: regularity, rugosity, ellipticity and elongation. By clustering of spermatozoa in subpopulations it was established that melatonin-treated group of bucks during fourth month of the experimental period had significantly higher proportion of spermatozoa with the longest tail and significantly lower proportion of spermatozoa with lowest size of middle part of tail in all parameters as well as during fifth month of the experimental period significantly lower proportion of spermatozoa subpopulation with the lowest size of head in all size parameters. By categorization of bucks according to proportion of motile spermatozoa it was established that the bucks with the highest motility of spermatozoa had spermatozoa with the longest tail and the highest values of middle part of tail parameters. Based on obtained data it can be concluded that the use of multivariate statistical analysis of clustering spermatozoa by morphometric parameters resulting in subpopulations of spermatozoa which were statistically different in proportion of particular subpopulations of spermatozoa between control and experimental groups. The positive impact of melatonin on abovementioned proportions of spermatozoa subpopulations was reflected with decreased proportion of spermatozoa subpopulation with the lowest size of head, proportion of spermatozoa subpopulation with the lowest size of middle part of tail and increased proportion of spermatozoa subpopulation with the longest tail in buck ejaculates. Higher motility of buck spermatozoa was influenced by longer middle part of tail and total length of tail. Thus, exogenous melatonin could be used for the improvement of spermogram in breeding bucks during non-breeding season.