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 crystalsof YBa2Cu3O7-
δ (Tc(P=0) ≈49.2K and δ ≈ 0.5). The critical temperature Tc is found to increase within creasing pressure a ta rate dTc/dP =+ −5.1 KGPa^-1, while ρ(300 K) decreases at a rate d ln(ρ)/dP= ( −19±0.2)%GPa^-1. Near Tc, 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 T0 determines the coherence length along the c-axis ξ (0) ≃(3.43±0.01)A at P=0, which is found to decrease withincreasing P. Atthesametime, Δ⁎ and the BCSr atio 2Δ⁎ /kB Tc 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 in dependent 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 behavioris observed forthe first time.