Electron microscopic investigations of surface spread synaptonemal complexes in spermatocytes from a 37-year-old man ascertained for infertility detected a pericentric inv(1), and subsequent lymphocyte analysis placed the breakpoints at p32 and q42. Most spermatocytes showed a maturation arrest at mid-pachytene explaining the azoospermia. As in two other comparatively large loop-forming pericentric inversions, initiation of synapsis took place in the middle of the inverted segment. Thus there is no indication of interstitial synaptic initiation being restricted to special pairing sites along the length of the chromosome. All spermatocytes investigated at mid-pachytene showed inversion loops, none of which was fully synapsed with a specific delay in pairing of the heterochromatic block 1qh and adjacent segments. The loops were of similar size in all the cells examined and synaptic adjustment had not taken place. There was no indication of a preferential association between the inv(1) bivalent and the XY configuration, and a functional disturbance of the X seems an unlikely reason for the meiotic maturation arrest. The most likely cause may be the failure of adequate synapsis of the inverted segment and the possibly associated pairing abnormalities of other homologues, including asynapsis and/or precocious desynapsis.