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The influence of hydrostatic pressure P up to 1.05 GPa on resistivity ρ , excess conductivity σ ′ ( T ) and pseudogap Δ ⁎ ( T ) is investigated in slightly doped single crystals of YBa 2 Cu 3 O 7− δ ( T c ( P = 0 ) ≈ 49.2 K and δ ≈ 0.5 ). The critical temperature T c is found to increase with increasing pressure at a rate dT c / dP =+ 5.1 KGPa − 1 , while ρ ( 300 K ) decreases at a rate d ln ρ / dP = ( − 19 ± 0.2 ) % GPa − 1 . Near T c , independently on pressure, σ ′ ( T ) is well described by the Aslamasov–Larkin and Hikami–Larkin fluctuation theories, demonstrating a 3D–2D crossover with increase of temperature. The crossover temperature T 0 determines the coherence length along the c -axis ξ c ( 0 ) ≃ ( 3.43 ± 0.01 ) A ˚ at P =0, which is found to decrease with increasing P . At the same time, Δ ⁎ and the BCS ratio 2 Δ ⁎ / k B T c both increase with increasing hydrostatic pressure at a rate d ln Δ ⁎ / dP ≈ 0.36 GPa − 1 , implying an increase of the coupling strength with increasing P . At low temperatures below T pair , the shape of the Δ ⁎ ( T ) curve is found to be almost independent of pressure. At high temperatures, the shape of the Δ ⁎ ( T ) curve changes noticeably with increasing P , suggesting a strong influence of pressure on the lattice dynamics. This unusual behavior is observed for the first time. [ABSTRACT FROM AUTHOR]