We have investigated the fertilisation competence, protein synthesis, histone H1 kinase and myelin basic protein (MBP) kinase activities in three categories of bovine oocytes (derived from three size categories of follicles: M--medium, 2.5-5.0 mm; S--small, 1.5-2.5 mm; T--tiny, 1.0-1.5 mm). In contrast to more or less normal meiotic maturation (85.6%) and fertilisation (70.8%) of M oocytes cultured for 24 h, the fertilisation of M oocytes cultured for 40 h was associated with increased rates of retarded male pronuclear development and retention of the second polar body. The S and T oocytes cultured for 24 h or 40 h were mostly arrested at defective late diakinesis-metaphase I (77.5-100%) stage. After fertilisation of S and T oocytes cultured for 24 h no polar body was extruded and formation of one, three or four female pronuclei, together with mostly normal male pronuclei, was observed. The fertilisation of S and T oocytes after 40 h culture resulted in a higher number of female and a decreased number of male pronuclei. A major change in the pattern of protein synthesis was associated with the resumption of meiosis. There were no significant differences in the profile of protein synthesis between oocyte categories in all groups either matured or fertilised. The H1 kinase activity reached comparable increased levels in oocytes of all categories matured for 24 h and decreased during the 40 h culture, most significantly in M oocytes. The MBP kinase activity was at approximately the same high level in all categories of oocytes after 24 h of culture and remained stable until 40 h. The fertilisation after 24 h of culture resulted, in M oocytes, in low levels of both H1 and MBP kinase activities; in S oocytes, only H1 kinase was completely inactivated while MBP kinase activity decreased to some extent; in T oocytes, both H1 and MBP kinase activity decreased. Fertilisation of all oocyte categories after 40 h culture resulted in complete inactivation of both these kinases to their basal levels.