Pinealocytes from adult guinea pigs contain conventional centrioles and microtubular sheaves composed of varying numbers of doublet and/or triplet tubules of indefinite length embedded in a dense matrix, which are assumed to be formed by elongation and dissociation of the centriolar wall. The observations suggest that one end of the wall of elongated centriolar formations may remain unbroken, while the other end may split into microtubular sheaves arranged in straight or skewed arrays. Additionally, maturing pinealocytes may possess two pairs of centrioles, of which some undergo lengthening and disintegration of the wall, giving rise to microtubular sheaves, while others remain unbroken. Cross-striated structures named denticulate or striped borders are often seen apposed to the elongated centriolar formations and are possible precursors of adjacent striated fibres.